They don't need WhatsApp's help to do it. WhatsApp voice calls are done over different ports, so only these need to be blocked.

The UAE (and Saudi Arabia) went through the extra step of asking WhatsApp to perform a block server side because of legal regulatory issues. Oman has made no such demand.

My family regularly makes WhatsApp calls to me from Oman; they just have to switch on a VPN to tunnel out of Oman's censorware.