I have no idea how the block works, but I learned that WhatsApp video calls are blocked in the United Arab Emirates, along with almost every other video chat app. There’s a UAE sponsored app called Totok that works, although it is/was removed from the Apple Store for privacy reasons. Recently they started allowing Zoom calls to go through.

Further reading:

https://www.cnbc.com/2020/03/26/coronavirus-lockdown-uae-res...

It's better to ask why Telegram was blocked, and I think it's often been because of their channels:

https://telegram.org/tour/channels

In this case it's more relevant to ask why Twitter or Facebook haven't been blocked (if they haven't).

I think WA has a similar feature ("broadcast") but not as widely used (since it is limited to 256 people).