WhatsApp/Viber calls gets disconnected when lock screen button is pressed

I have been having trouble since I upgraded to ios10.0.1 & now 10.0.2 and whatsp app 2.16.10. When I press the lock screen button when I am on a whatsapp audio call the call gets disconnected. This happens even after changing the phone(6->7)/reinstalling whats app. The same problem happens with viber too.

Seems like an ios10 issue but not sure why its just happening to few users. My friends are on the same versions and it works fine.

PS: Interestingly the call doesn't get disconnected when I use bluetooth headset and press the lock screen button.

Looking forward to some help to get this resolved.

Thanks

-Magizh