"You cannot place a WhatsApp call if youre already on another call"

I have and iPhone7 software 14.6

- I keep getting this message when I want to place a call - despite the fact that I have already completed the previous call. "You cannot place a WhatsApp call if youre already on another call "

This problem occurs more frequently when the call disconnects because of poor wifi.

It takes several instances of power off / on my phone and swipping up all my apps before I can make a call - this could be hours sometimes!!

I have 'local network' already selected