Does it do this when you're using the speakerphone or just when you have the phone held next to your head?
If the latter, does the screen stay on while you're talking/listening (you may want someone else to check) or does it go off/black (which is what it should be doing, until you move it away from your head)? If the former, then it sounds like something is interfering with the proximity sensor, and then you brush against the screen and the phone app gets minimized. You may want to check your screen protector or case to be sure they're not intruding on the proximity sensor.
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