Please help… Gadwell UK uses very strange settings for SIP trunking, like anonymous SIP calls, no registration, etc. I manage to set all of that up, calls are coming in and out, with correct caller ID and DDIs, however, there is silence on both ends 
Many thanks for any help!
P.S. All the ports are opened, tried with firewall off, dmz, etc. Same result
@pvladi It would help to see the settings they want you to use and even a SIP debug of the call. It’s sounding like RTP isn’t being passed or if it is, it’s being blocked somewhere.
Have you set the NAT settings correctly on your FPBX with your external IP?
Also if you continue to have issues then get them to set you up an IAX2 trunk and test with that.
I have an IAX2 trunk with gradwell and the config is very simple.