calls softphone to softphone not working

[tomtom78]

Hello

I am using an OXO with softwareversion 40.001 with IP Desktop Softphone Version 12.1.1. My IP Desktop Softphones on windows 10 and MacOS computers have an issue with internal phone calls. When a softphones calls another softphone the call is signalised and I can take the call. After the call is taken, I can not hear any sound from both sides.
When I make a call to a IP Desktop Phone 8068s then it is working without problems.

I also made a Wireshark trace and saw that there is no UDP traffic from softphone to softphone.

To solve the problem I made the following steps:
- deactivate antivirus
- deactivate windows firewall
- installed all windows updates
- detached headset
- uninstalled any unneeded software
- deactivated ipv6

But still no success.

If someone has a hint what else I can check, I would be very glad

Cheers Tom