To record an active call, tap "More" and select "Start Recording." A blinking red dot will appear next to the call timer. To end the recording, simply "End Call" or select "Stop Recording."
You can listen to your Recorded Calls in the Call History entry for the call.
The recording is saved in the WAV format, and can be found with a File Explorer of your choice. If you have an SD card, it can be found in the "/sdcard/Bria_Mobile" folder. If you do not have a card, it can be found in "/storage/emulated/0/Bria_Mobile/" or a similar path.