No audio on internal calls

Hello!

I got a freepbx (FreePBX 17.0.19.24, Asterisk Version: 21.5.0) here
It’s running for 3 months now. So far we did not have any issues.
Today I we got the following problem:
There is no audio on internal calls in both directions. If you put the call on hold for a second audio starts working.
The issue only occurs if both participants use a snom wired device (We have 50% Snom wired devices and 50% Snom DECT phones attached by Snom M900 IP Dect Stations)
So if one participant uses a DECT phone everything is fine.
No problems if it is an oubound or inbound call.

I will do a restart of the VM running freepbx and asterisk tonight.

Is there anything I can do to narrow it down?

Thanks in advance.

Gerald