We now have the following call integration modes to help improve the calling experience
Default - Incoming Mobile calls will not interrupt an existing VoIP call. You can then swap between the Mobile call and VoIP call
System UI - Dialling an outbound VoIP call or receving an Inbound VoIP call will now use the native call interface. This allows the best interoperability with the system regarding Bluetooth and other audio devices.
Please note this feature requires that the manufacturer has support for it. Although the latest Android OS versions support this feature from API level 23, not all device manufacturers have fully implemented this API so if there are issues you will need to change to one of the other modes. Before doing this, check in the 'Settings' application for the device that your app has the 'Phone' permission enabled and also ensure the 'Calling Account' is enabled when shown the respective dialog screen.
Disabled - Only use this mode if you have issues receiving Incoming calls with the other two modes. Incoming Mobile calls will interrupt existing VoIP calls and put them on hold.