I am using Android native sip client - works perfectly over WiFi - both local and remote
Over 4G - I can make calls properly - but incoming calls ring, but am unable to answer - just keeps ringing.
Are there any tweaks to FreePBX I can make to try and fix the problem.
(I have set up VPN back to FreePBX - all works perfectly over 4G - but have to keep resetting the client to re-connect)
I want to use the native client as any other clients (that I have found) do not allow answering via bluetooth button on car hand free system, which is required when driving - unless anyone knows of Sip client that allows that.
Check with your 4G provider and see if you can talk to a Tier 2 Tech Support person. Ask them about restrictions they place on 4G and SIP. I think that will help.
Secondary to that - change the port on your App to receive calls on 6050 (or 1234, of whatever) to get it off the 5060 port. They could be filtering by port, in which case you’re boned.
Finally, make sure the NAT setup for your application is correct. It’s possible that you are losing the connection “in transit”. A SIP DEBUG of the problem (from the server is fine) would help you identify that, and you’ve got the technology to make that happen.