Description of the issue: If there is an incoming call on the not locked display (application in the background) nad you are using Bluetooth headset in the same time, the audio of the call after the answer is coming from the speaker. While you switch in manualy in teh applciation there is no audio at all.
Solution: By default apple call kit which is the API interface for the application which requires the access for the calling functions has speaker output defined as default one. The apple call kit is rejecting to use some types of headsets. This behaviour is possible to change in the SETTINGS->GENERAL->ACCESSIBILITY->CALL AUDIO ROUTING.