Hi
If you need to check my phpfox experience, read my reviews, they are mostly for various phpfox mods and fixes. As I understand you want a module to ask phone number on registration from user, then user recieve varification code on phone number and enter it on step 2 to verify phone. I get it right?
Its not a problem to do, and really depends only on sms gateway you want to use. Twilio is easy to integrate, if you want to use something else, tell me, I will check API documentation.
Best Regards, Yura