Whitelisting VoIP IP addresses Firewall

Hello

I am having issues with intermitent one way audio problems with VoIP. I am using the latest merlin firmware on a RT-AX82U router thanks to gnutons port. I am using a Grandstream HT801 ATA with an analogue Dect Phone BT8600
I have a FTTP connection that is CGNAT enabled and getting a Public IPv4 IP address is not an option (and should not be needed with a SIP Connection)

I am new to Asus routers and I have read on the website of our VoIP provider that it is strongly advised to...

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