Add not_number translation

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?
 

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: