Calling iOS App

Hello

We have installed our own home server to test matrix synapse and riot and have a problem with the iOS app.
Every time someone is called on the iOS app, the called person does not hear anything.
This happens if the caller is using the Android, the iOS or the web application.
If the person calls someone from the iOS app to the Android or web app, it works fine.
All other communication between the desktop client, Android and web app works fine, so it seems to be some problem with the iOS app.
Is this a new or known bug or do we need to allow something specific on the iOS devices?

We are using the latest version of the Synapse homesever with coturn and the riot iOS app. iOS Version 13.3

Best regards
Dennis