How do I silence incoming voice calls on WhatsApp?

I've been forced by my relatives to install WhatsApp on my iPhone. I use it only for text messages, so I disabled camera, voice, etc. and, what I thought, was all sounds from it in the iPhone settings.

But once in a while, one of my older relatives decides to make a voice call to me. In that case the WhatsApp app starts playing this annoying ringing which I cannot kill -- that is very frustrating especially if I'm at work.

Is there a setting (or some other way) to prevent it from ringing the incoming voice calls?

PS. I do not want to silence my phone completely. Just WhatsApp.