I can enter almost any number format in there, it seems. So, why display this text underneath the field as instruction (requirement?) to the user if it's not required? Requiring that rigid format would be silly anyway, so I just wonder why that is under the field?
I have tried 8885551212 and 888-555-1212 and both worked.
I've mentioned the space after the area code closing paren before:
Now after entering data a few times, the format suggestion displayed is apparently ignored so why display that to the visitor anyway? Just let them enter their phone number.