diff --git a/app/models/user.rb b/app/models/user.rb
index 0ca6d0944..3d6dbe893 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -19,7 +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.shared.numeric.format", field: "") }
+  validates :phone_extension, format: { with: /\A\d+\z/, allow_blank: true, message: I18n.t("validations.not_number", field: "") }
 
   after_validation :send_data_protection_confirmation_reminder, if: :is_dpo_changed?
 
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 05dda8f7b..6867cb804 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -255,6 +255,7 @@ en:
     not_answered: "You must answer %{question}"
     invalid_option: "Enter a valid value for %{question}"
     invalid_number: "Enter a number for %{question}"
+    not_number: "%{field} must be a number."
     no_address_found: "We could not find this address. Check the address data in your CSV file is correct and complete, or select the correct address using the CORE site."
 
     date: