Accessing Control Center Iphone XS

Not sure what you mean by a separate 'audio button' or where it is. When I get a call, sometimes, an audio output list (bluetooth/speaker/iphone) comes on when I answer, in which case I just pick whatever audio device I wish to use. But sometimes the audio output list does not come up -- and then I I'll need to access audio output options by swiping down on top right to get to the control center and choosing the circle/arrow audio option and then the device (bluetooth). The bluetooth is already connected to the iphone so connecting it is not the answer. That's what I'm referring to above. The swipe does not always work.

Thanks for your help. The audio button was the solution. I just don't set up stuff while I'm actually IN a call...I try do it before or after and you can't access or even see that option unless you're actually talking. But as for the green bar -- it did not respond to tapping. And I am adding to this post that if anyone else has this issue they need to search for 'switch audio output while on phone' to get an answer...otherwise the only thing that shows up in a search is the control center solution.