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?