diff --git a/src/Form/ModuleConfigurationForm.php b/src/Form/ModuleConfigurationForm.php
index 825eb45dc6db1b9307f9b12b83546f0e623bef03..66d4b08089896e8c8fec03b9718fe2954d2a482b 100644
--- a/src/Form/ModuleConfigurationForm.php
+++ b/src/Form/ModuleConfigurationForm.php
@@ -215,6 +215,7 @@ class ModuleConfigurationForm extends ConfigFormBase {
     return parent::buildForm($form, $form_state);
   }
 
+
   /**
    * {@inheritdoc}
    */
@@ -250,4 +251,23 @@ class ModuleConfigurationForm extends ConfigFormBase {
 
     parent::submitForm($form, $form_state);
   }
+
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $values = $form_state->getValues();
+
+    // phone numbers
+    $phoneNumbers = ['phone', 'fax'];
+
+    foreach ($phoneNumbers as $number) {
+      if (strlen($values[$number]) < 7) {
+        $form_state->setErrorByName($number, $this->t('The ' . $number . ' number is too short.'));
+      }
+    }
+
+  }
+
 }