Sony Z3, D6603, Microphone icon is muted on Whatsapp phone calls

I can record Whatsapp audio messages OK but as soon as I make a Whatsapp phone call the microphone icon is muted and people cannot hear me.

Microphone works OK for non Whatsapp phone calls.

Any suggestions welcomed

Thanks

I decided to add some images!
The first image shows my microphone icon when I use WhatsApp normally

The second image shows the muted microphone icon when I try to make a WhatsApp phone call!!

When people answer my phone call they do not hear me; I did not find any way to "unmute" my mike; suggestions welcomed