I agree that this should be fixable in the Settings, and it is a truly annoying "feature". Stupid work-around: While on the call, press the "Recents" virtual button to display your recently opened apps. This will allow your device "Screen timeout" setting to over-ride the short phone timeout non-setting. Muting/unmuting, ending call, etc. will require one extra tap on the screen, but that's simpler (for me) than unlocking the phone with a fingerprint, especially if the phone is on a charging pad.
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