MGCP IP Phone and no audio

We have an IP phone system that we are trying to get up and running on our network.

We've created the VIP with forwarding from the phone systems external IP to our internal IP and opened the necessary ports as well.

We've added it to the firewall policy and have the phone and see and ring each other on the network, but the call will connect with no audio on both sides. We added the phone systems external IP as a secondary IP on our Wan1 interface. This gave use audio on internal office calls, but anything outside the LAN still has no audio.

Any ideas would be great. I'm not a networking person, but unfortunately, trying to fill the role.

Thanks