Welcome to Android Central! Which Stylo phone? There are 6 generations.
Are you certain you're not accidentally tapping the Mute button while you're on the call? Do you notice that the screen is still on while the phone is up against your ear? If so, then that means the proximity sensor isn't working correctly, making it easier for your face to accidentally tap onscreen buttons while on a call.
Also, does this only happen when you're using a headset while on the call?