I have a customer using freepbx with sip client on their cell phone. Grandstream wave stopped working. So we tried Zoiper and it works, but not on mobile data / cellular data. This is the only user with this problem (Iphone on Tmobile) The extension is the PJ_SIP on freepbx, port 5060. Would it make sense to switch that extension to chan_sip on port 5160? Is it possible Tmobile is blocking sip traffic /registration?

I've heard of Tmobile causing problems for VoIP - more to do with IPv6 behind the scenes if I remember correctly. Any chance your client could VPN their cell phone to the office and work around the problem that way?

chan_sip probably wouldn't help anything...

The Freepbx instance is in the cloud, I am not too interested in setting up VPN on the cell phone etc. I will test using an alternate port or IAX as a last resort. Maybe we can bypass whatever block is there.

IAX using Zoiper seems to be more reliable and possibly bypasses whatever is in the way. Anyone have an opinion why I should't be using IAX instead of SIP.

IAX is probably a good work around. All the information passes on the same port number (unlike SIP) which usually improves NAT traversal.