I figured out that the RTP connection was not established because of the way I was testing the system out. I was testing things out by calling into the PBX using a (Vonage) VoIP line that was on the same private network. The external line connected to the PBX is a different (Softphone) line from Vonage. So I would call from my home line, it would go out to the Vonage server to resolve the number, and come back into my private network where the PBX is setup on a PC behind a NAT router. Then when the call was transferred to an outside extesion, outside the private network, there was no audio. My knowledge of RTP is limited, but I'm guessing my router had some kind of problem with what was going on (Maybe it cannot have two outgoing RTP streams simultaneously). So after figuring this setup might be a problem, I called into the PBX from a land line and everything was fine.
There is no problem with relaying RTP from inside a NAT or through a NAT, as long as the public IP address is found using UPnP, STUN, or static.