How to lock the screen when calling with whatsapp iphone 6+ ios 10.0.2

When I'm calling on whatsapp I cannot any more lock the screen and thus save the battery.

Iphone 6+ with IOS 10.0.1 AND 10.0.2

Indeed if I try to lock the screen it ends the whatsapp phone call...with IOS 9 it was not the case : I could lock the screen and pursue my phone call on whatsapp.

WHY?????? (unhappy with this new ios)