Talk Snap Coturn - no sound

We have a nextcloud snap installation (16.0.5) with nextcloud talk on a cloud server.
On a different cloud server, we have a coturn server.
The coturn server is registered in the nextcloud talk settings; the check of the coturn server in the talk settings results ‘ok’.
When trying to call my wife (Iphone w/ IOS 13.2.2 to Iphone w/ IOS 13.2.2) using nextcloud talk we get two black screens and no sound.
What can I do?