Problem description
We cannot use MS Teams and Ringotel simultaneously. If we are in a Teams meeting and receive a call on Ringotel, the Teams meeting is terminated. Similarly, if we are using Ringotel to communicate with a customer and someone calls us via Teams, the Ringotel call is disconnected.
Explanation
It turn out to be a known issue with MS Teams and VoIP applications. You can check, for example, this thread for a number of similar complains with other VoIP applications: https://answers.microsoft.com/en-us/msteams/forum/all/teams-call-will-drop-when-i-receive-a-call-from-a/b589bbe5-505a-46ec-9b0c-fc956d8f9620
I can advise to address this issue directly to MS Teams support.