diff --git a/application/language/arabic/translations_lang.php b/application/language/arabic/translations_lang.php
index a0e907c1..0de1b724 100755
--- a/application/language/arabic/translations_lang.php
+++ b/application/language/arabic/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/bulgarian/translations_lang.php b/application/language/bulgarian/translations_lang.php
index ba1eba30..ec3879fc 100755
--- a/application/language/bulgarian/translations_lang.php
+++ b/application/language/bulgarian/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/catalan/translations_lang.php b/application/language/catalan/translations_lang.php
index e80a8b48..234c3d05 100644
--- a/application/language/catalan/translations_lang.php
+++ b/application/language/catalan/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/chinese/translations_lang.php b/application/language/chinese/translations_lang.php
index 90f8b55c..6f651d1f 100755
--- a/application/language/chinese/translations_lang.php
+++ b/application/language/chinese/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/czech/translations_lang.php b/application/language/czech/translations_lang.php
index 868aaa04..4fb900ae 100644
--- a/application/language/czech/translations_lang.php
+++ b/application/language/czech/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/danish/translations_lang.php b/application/language/danish/translations_lang.php
index 50f299c5..aac6e2ef 100755
--- a/application/language/danish/translations_lang.php
+++ b/application/language/danish/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/dutch/translations_lang.php b/application/language/dutch/translations_lang.php
index 98bca284..033eeebc 100755
--- a/application/language/dutch/translations_lang.php
+++ b/application/language/dutch/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/english/translations_lang.php b/application/language/english/translations_lang.php
index 1c41bd41..2b3cc7d4 100755
--- a/application/language/english/translations_lang.php
+++ b/application/language/english/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/finnish/translations_lang.php b/application/language/finnish/translations_lang.php
index c81697fe..655c9623 100755
--- a/application/language/finnish/translations_lang.php
+++ b/application/language/finnish/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/french/translations_lang.php b/application/language/french/translations_lang.php
index 9b181bf7..a13246d6 100755
--- a/application/language/french/translations_lang.php
+++ b/application/language/french/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/greek/translations_lang.php b/application/language/greek/translations_lang.php
index 8372360d..553ee2f5 100755
--- a/application/language/greek/translations_lang.php
+++ b/application/language/greek/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/hebrew/translations_lang.php b/application/language/hebrew/translations_lang.php
index f7762590..1406f2d6 100644
--- a/application/language/hebrew/translations_lang.php
+++ b/application/language/hebrew/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/hindi/translations_lang.php b/application/language/hindi/translations_lang.php
index 7f9be4be..fa873f6e 100755
--- a/application/language/hindi/translations_lang.php
+++ b/application/language/hindi/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/hungarian/translations_lang.php b/application/language/hungarian/translations_lang.php
index ce9b63fd..1679b7d7 100755
--- a/application/language/hungarian/translations_lang.php
+++ b/application/language/hungarian/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/italian/translations_lang.php b/application/language/italian/translations_lang.php
index 0b40ba8e..5c9b75d6 100755
--- a/application/language/italian/translations_lang.php
+++ b/application/language/italian/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/japanese/translations_lang.php b/application/language/japanese/translations_lang.php
index 1655c6b9..e09b4f74 100755
--- a/application/language/japanese/translations_lang.php
+++ b/application/language/japanese/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/luxembourgish/translations_lang.php b/application/language/luxembourgish/translations_lang.php
index b55f74b1..eae96d0f 100755
--- a/application/language/luxembourgish/translations_lang.php
+++ b/application/language/luxembourgish/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/marathi/translations_lang.php b/application/language/marathi/translations_lang.php
index 65110e70..94061e13 100644
--- a/application/language/marathi/translations_lang.php
+++ b/application/language/marathi/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/polish/translations_lang.php b/application/language/polish/translations_lang.php
index 61cad48e..883746f9 100755
--- a/application/language/polish/translations_lang.php
+++ b/application/language/polish/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/portuguese-br/translations_lang.php b/application/language/portuguese-br/translations_lang.php
index 60c45a53..9fc39f7a 100755
--- a/application/language/portuguese-br/translations_lang.php
+++ b/application/language/portuguese-br/translations_lang.php
@@ -337,4 +337,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/portuguese/translations_lang.php b/application/language/portuguese/translations_lang.php
index 89278651..4dcbe1f7 100755
--- a/application/language/portuguese/translations_lang.php
+++ b/application/language/portuguese/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/romanian/translations_lang.php b/application/language/romanian/translations_lang.php
index 773c1b08..3ea44bbb 100755
--- a/application/language/romanian/translations_lang.php
+++ b/application/language/romanian/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/russian/translations_lang.php b/application/language/russian/translations_lang.php
index e704b93a..3899e232 100755
--- a/application/language/russian/translations_lang.php
+++ b/application/language/russian/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/slovak/translations_lang.php b/application/language/slovak/translations_lang.php
index 1837d6c2..157dc4c5 100755
--- a/application/language/slovak/translations_lang.php
+++ b/application/language/slovak/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/spanish/translations_lang.php b/application/language/spanish/translations_lang.php
index d9ece108..d9da6dee 100755
--- a/application/language/spanish/translations_lang.php
+++ b/application/language/spanish/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/swedish/translations_lang.php b/application/language/swedish/translations_lang.php
index 058b6046..229310a0 100644
--- a/application/language/swedish/translations_lang.php
+++ b/application/language/swedish/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/application/language/turkish/translations_lang.php b/application/language/turkish/translations_lang.php
index a061e24d..65f99707 100755
--- a/application/language/turkish/translations_lang.php
+++ b/application/language/turkish/translations_lang.php
@@ -338,4 +338,5 @@ $lang['display'] = 'Display';
 $lang['require'] = 'Require';
 $lang['color'] = 'Color';
 $lang['matomo_analytics_url_hint'] = 'Add the URL to your own Matomo installation to enable Matomo tracking on the booking pages.';
+$lang['invalid_phone'] = 'Invalid phone number.';
 // End
diff --git a/assets/js/pages/admins.js b/assets/js/pages/admins.js
index 82656989..9337dba0 100644
--- a/assets/js/pages/admins.js
+++ b/assets/js/pages/admins.js
@@ -292,6 +292,22 @@ App.Pages.Admins = (function () {
                 throw new Error(lang('invalid_email'));
             }
 
+            // Validate phone number.
+            const phoneNumber = $phoneNumber.val();
+
+            if (phoneNumber && !App.Utils.Validation.phone(phoneNumber)) {
+                $phoneNumber.addClass('is-invalid');
+                throw new Error(lang('invalid_phone'));
+            }
+
+            // Validate mobile number.
+            const mobileNumber = $mobileNumber.val();
+
+            if (mobileNumber && !App.Utils.Validation.phone(mobileNumber)) {
+                $mobileNumber.addClass('is-invalid');
+                throw new Error(lang('invalid_phone'));
+            }
+
             // Check if username exists
             if ($username.attr('already-exists') === 'true') {
                 $username.addClass('is-invalid');
diff --git a/assets/js/pages/booking.js b/assets/js/pages/booking.js
index 81778127..632a73c5 100644
--- a/assets/js/pages/booking.js
+++ b/assets/js/pages/booking.js
@@ -552,6 +552,14 @@ App.Pages.Booking = (function () {
                 throw new Error(lang('invalid_email'));
             }
 
+            // Validate phone number.
+            const phoneNumber = $phoneNumber.val();
+
+            if (phoneNumber && !App.Utils.Validation.phone(phoneNumber)) {
+                $phoneNumber.addClass('is-invalid');
+                throw new Error(lang('invalid_phone'));
+            }
+
             return true;
         } catch (error) {
             $('#form-message').text(error.message);
diff --git a/assets/js/pages/customers.js b/assets/js/pages/customers.js
index 6a9421a0..e0e5f562 100644
--- a/assets/js/pages/customers.js
+++ b/assets/js/pages/customers.js
@@ -213,11 +213,21 @@ App.Pages.Customers = (function () {
             }
 
             // Validate email address.
-            if (!App.Utils.Validation.email($email.val())) {
+            const email = $email.val();
+
+            if (email && !App.Utils.Validation.email(email)) {
                 $email.addClass('is-invalid');
                 throw new Error(lang('invalid_email'));
             }
 
+            // Validate phone number.
+            const phoneNumber = $phoneNumber.val();
+
+            if (phoneNumber && !App.Utils.Validation.phone(phoneNumber)) {
+                $phoneNumber.addClass('is-invalid');
+                throw new Error(lang('invalid_phone'));
+            }
+
             return true;
         } catch (error) {
             $formMessage.addClass('alert-danger').text(error.message).show();
diff --git a/assets/js/pages/providers.js b/assets/js/pages/providers.js
index b6acf445..0b0cdf17 100755
--- a/assets/js/pages/providers.js
+++ b/assets/js/pages/providers.js
@@ -300,6 +300,22 @@ App.Pages.Providers = (function () {
                 throw new Error(lang('invalid_email'));
             }
 
+            // Validate phone number.
+            const phoneNumber = $phoneNumber.val();
+
+            if (phoneNumber && !App.Utils.Validation.phone(phoneNumber)) {
+                $phoneNumber.addClass('is-invalid');
+                throw new Error(lang('invalid_phone'));
+            }
+
+            // Validate mobile number.
+            const mobileNumber = $mobileNumber.val();
+
+            if (mobileNumber && !App.Utils.Validation.phone(mobileNumber)) {
+                $mobileNumber.addClass('is-invalid');
+                throw new Error(lang('invalid_phone'));
+            }
+
             // Check if username exists
             if ($username.attr('already-exists') === 'true') {
                 $username.addClass('is-invalid');
diff --git a/assets/js/pages/secretaries.js b/assets/js/pages/secretaries.js
index d4928900..450e3f1f 100644
--- a/assets/js/pages/secretaries.js
+++ b/assets/js/pages/secretaries.js
@@ -305,6 +305,22 @@ App.Pages.Secretaries = (function () {
                 throw new Error('Invalid email address!');
             }
 
+            // Validate phone number.
+            const phoneNumber = $phoneNumber.val();
+
+            if (phoneNumber && !App.Utils.Validation.phone(phoneNumber)) {
+                $phoneNumber.addClass('is-invalid');
+                throw new Error(lang('invalid_phone'));
+            }
+
+            // Validate mobile number.
+            const mobileNumber = $mobileNumber.val();
+
+            if (mobileNumber && !App.Utils.Validation.phone(mobileNumber)) {
+                $mobileNumber.addClass('is-invalid');
+                throw new Error(lang('invalid_phone'));
+            }
+
             // Check if username exists
             if ($username.attr('already-exists') === 'true') {
                 $username.addClass('is-invalid');
diff --git a/assets/js/utils/validation.js b/assets/js/utils/validation.js
index 84a8e48b..3257355c 100644
--- a/assets/js/utils/validation.js
+++ b/assets/js/utils/validation.js
@@ -29,7 +29,21 @@ window.App.Utils.Validation = (function () {
         return re.test(value);
     }
 
+    /**
+     * Validate the provided phone.
+     *
+     * @param {String} value
+     *
+     * @return {Boolean}
+     */
+    function phone(value) {
+        const re = /^[\+]?[(]?[0-9]{3}[)]?[-\s\.]?[0-9]{3}[-\s\.]?[0-9]{4,6}$/im;
+
+        return re.test(value);
+    }
+
     return {
-        email
+        email,
+        phone
     };
 })();