Problem:
Format custom Phone field with Number Format (999) 999-9999 or 999-999-9999

Solution:
The number field will not allow to save the dash - or brackets () as it's a numeric field.

If you want to save the - or () with your number field, you need to change your phone/fax field type to text (single text field) and then we can use the Toolset form's hook: cred_before_save_data
=> https://toolset.com/documentation/programmer-reference/cred-api/#cred_before_save_data

You can find the proposed solution in this case with the following reply:
=> https://toolset.com/forums/topic/phone-number-format-999-999-9999-or-999-999-9999/#post-1975289

Relevant Documentation: