Fix/troublehoot 3CX v20 VoIP Issues

After moving our PBX to 3CX v20 every Grandstream HT801 on the network suddenly stopped handling calls—no one can dial in or out, even though the devices register and the network settings have been double-checked. I’m looking for someone who knows 3CX v20 inside out, is comfortable digging through activity logs, SIP traces and Grandstream provisioning files, and can quickly pinpoint what changed in the upgrade that now blocks audio or signalling.

The job ends when every HT801 can place and receive external calls reliably and I have a short write-up of the root cause and the changes you made (so we can avoid the issue next time). Wireshark, 3CX Management Console, and any Grandstream-specific tools are all fair game—use whatever gets this fixed fast.