Trying to get NAT working on a Polycom 330 phone using the 3cx phone software.

Here is the scenario. We have a 3cx server which is at our main office behind NAT. We are looking to deploy VOIP phones in district classroom sites which will be behind other school district NATs. We have purchased a Polycom Ip 330 phone and have it set up and working perfectly on our internal system with the 3cx server.

When we place one of these in an external classroom site I am able to call out from the phone and connect and talk no problem. When I have someone call the phones extension on the 3cx system it rings the phoen at teh external classroom site but it will not allow me to pick up the call and answer it. I am assuming this is because of the NAT translation at the external classrooms.

Trying to research this I see we can use a stun server to help NAT translation, but the issue is Polycom of course doesn't support STUN. I'm glad they made this easy!!!! So contacting Polycom's support they just directed me to the manual which has a few things regarding NAT and not much help at all.

What suggestion could be made for my scenario? Since the SIP connection works but the RTP voice connection seems to have issues when traveling through a NAT and then having to travel through another Nat to hit the phone server. Has anyone ecountered this scenario using Polycom VOIP phones? Would it be better to purchase a SNOM phone that lets us use STUN server settings to help translate both NATS? Any suggestions or help would be appreciated because Polycom wasn't much help at all.