Blockin Whatsapp Videocall only

Hello Everyone! 

I didnt found a similar post , so here I come.

We got a Fortigate 600E at our company and we are trying to block Whatsapp Videocall only (voice call, text and file transfer should work). I know there exist Whatsapp signatures in app-control, but I can't figure out how to disable only video without affecting voice. 

Is there any workaround to only block Video calls? No matter if is using firewall policies, app control, etc. 

Thanks!!