Once confident, run the firewall checker and ensure that it passes with flying colours.
Then, if you really want to limit the port forward to specific external IP addresses, ensure that all SIP and RTP ports are allowed from our distributed SIP trunking system called GoVoIP.
The easiest way to do this is to allow all IP addresses for our hosted services which is currently 103.58.228.0/22 or if you want to be very specific then just allow 103.58.228.32/28 and 103.58.229.32/28