When you have configured validation rules for the phone numbers.
A user will see automatically an error message when the phone number is not according to the right format.
See below for a screenshot of an example error message:
At the configuration you can specify which formats you will allow.
The validation will happen on the following places:
- Contribution Page
- Event Registration Page
- Petition Registration Page
- Profile forms
- Contact forms, such as the edit contact and the inline edit phone.