What kind of validation rules SFMC uses when imports phone numbers?

Question

Do you know what kind of validation rules SFMC uses when imports phone numbers? Do you have experience with with phone masking for test environments and what phone mask is advised? We are concerned that if we will append phone number with character or simple change it's length - phone number will be rejected in SFMC.

Answer 1

Below is the validation they do according to the documentation:

Phone - A phone number
This data type accepts any string with 15 digits in it and strips out non-digit characters. For example, these examples are acceptable:

555-555-5555
1-555-555-5555
1-(555)-555-5555
(555)-555-5555

I do not believe there is anything beyond above, but if there is, it is like list detective and is not defined externally.