IPFW Dropped Whatsapp voice calls and router with ipfw

[Dmitry D]

Hello,

I have a small office network with private IP numbers.
FreeBSD router with nat + ipfw connected to Internet

I nave a problem with Whatsapp voice calls to smartphones in my LAN
Phone ringing, but call dropped after answer, or second case -
remote caller hear long beep while smartfone in LAN answer a call.

What ports I need to open in ipfw?
Thank you!

Dmitry.