Hello Sascha,
If all users are using same business roles, frontend settings/situations are more suspicous.
1. Would you please check what applications are being used when a call comes in. Does the issue also happen to some users even when the same applications are being used?
2. If possible, please let a user change computer to check it. For example, the function works fine on user A's computer, but not work on user B's computer. Please let user B use User A's computer and logon the CRM using user B's own credential. Also, please let user A use user B's computer and logon the CRM using user A's own credential.
Best Regards, Corrine