The "voice settings" menu from SFB app (Android and IOS) should be implemented in Teams app as well. There you can choose whether calls to cellphone should be VOIP, VOIP on WiFi only or Cellular. In the Scandinavian countries operator-based forking is common, and this setting improves the integration between the operator and Teams. There could also be a setting "VOIP for Teams calls, Cellphone for PSTN calls". Also, make these settings configurable from O365 admin portal and/or Powershell.
John Hansen commented
You could even add new options currently not avaialble on Skype such as "Suppress Teams call alert on mobile in case of active cellular call"