WhatsApp calls silenced

Hello. I have a problem with WhatsApp. I have emailed them but no answer, just redirected me to their FAQ.
The problem is, when my phone is in my bag and I receive a WhatsApp call, it rings for around 4 seconds, and it mutes but keeps ringing. No sound. When the phone is in the open, facing up, screen not covered, it rings for the whole time.
I thought it's because of Android's easy mute, where you hover your hand over and it mutes. So I turned that option off. But still it is the same. I don't know what else could be causing it.
I live in a foreign country and my whole communication with my family and friends, who are back at home, is over WhatsApp. And because of this, I never hear the call when I'm outside. I'd appreciate any help.