Phone Number with Extension

I'm currently using the survey platform and trying to gather basic identification information from the survey takers with a "Text Entry - Form" question type and requiring a response (force response). The information I'm asking in this question are:

* first name
* last name
* email (using valid email address validation)
* phone number (using valid phone number - US phone number validation)

The problem I'm running into is that some of the survey takers have a phone extension and some don't. So when I add a text entry field for "Extension", and require a response (for the entire question) with force validation, the survey takers who DON'T have a phone extension will get held up.

What is the best way of handling this type of situation? Surely, I cannot be the first person who has run into this problem. I just couldn't find any articles to diagnose the problem on my own.

Thanks in advance!
Kelsey