Okay, I still have had no joy in getting this sorted.
So, taking into account my limited knowledge of VOIP, this is my basic set up.
All phones and UCM 6202 using the latest firmware.
Static IP address (just used for the UCM) going to a Watchguard Firebox.
The rule on this Firebox is set as Any from all sources. IE any data coming in on a port is routed to the UCM on the same port and likewise from the UCM to the internet. Intrusion detection on the Watchguard is switched off.
I have two SIP accounts registered on the UCM (sipgate.co.uk).
Currently I have three phones registered to the UCM on three extensions.
Two of the phones are Snom (765 and M25 DECT) and the third is a GS WP820 (also have a 2170, but not using this whilst trying to troubleshoot).
All phones are fine when making outbound calls.
The Snom phones are fine when receiving inbound calls.
When the GS WP820 receives a call (and the GS2170 when it was connected before my last factory reset), the calling party cannot hear any audio. This problem only happens for calls coming in from the VOIP trunk. Internal extension - extension calls are fine.
The WP820 has been set up using the GS Zero Config option and I have also tried a manual configuration set up. Both result in the same issue.
I have tried a packet filter, but don’t have Wireshark or any reader that can open the .pcap file.
Help would be appreciated.