Here is your answer.

1. Voipo provides a stun server which helps with nating issues. zeus.voipwelcome.com
2. If that dont work here are ports you need:

a. 5060-5061 (at least) UDP / BIDIRECTIONAL: These are SIP registration ports for you to register to the server.

b. 10000-20000 UDP BIDIRECTIONAL: These are the ports SIP communication happens over.

This should resolve nating issues. Also 1 to 1 nat is ideal, if not possible then do port forwarding of all these ports inbound to your ata and allow all out from your ata.

Side note none of my clients that are using Voipo devices have issues with NAT. They plug and play voipo devices are are golden. I believe this is due to zeus stun server. I have about 50 clients using voipo devices the rest are using their sip credentials in pbx systems and softphones.