diff --git a/app/models/user.rb b/app/models/user.rb
index 2b1c5b9ca..ca8233980 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -19,6 +19,7 @@ class User < ApplicationRecord
   validates :password, presence: { if: :password_required? }
   validates :password, length: { within: Devise.password_length, allow_blank: true }
   validates :password, confirmation: { if: :password_required? }
+  validates :phone_extension, format: { with: /\A\d+\z/, allow_blank: true, message: I18n.t("validations.numeric.format", field: "") }
 
   after_validation :send_data_protection_confirmation_reminder, if: :is_dpo_changed?