This may shed some light for those who are trying to estabilish VOIP behind a NAT router. - RM
from
The Trouble with NAT and VOIP "In addition, the way in which conventional VoIP protocols are designed is also posing a problem to VoIP traffic passing through NAT. Conventional VoIP protocols only deal with the signalling of a telephone connection. The audio traffic is handled by another protocol and to make matters worse, the port on which the audio traffic is sent is random. The NAT router may be able to handle the signalling traffic, but it has no way of knowing that the audio traffic is related to the signalling and should hence be passed to the same device the signalling traffic is passed to. As a result, the audio traffic is simply discarded.
"At first, for both the calling and the called party everything will appear just fine. The called party will see the calling party's Caller ID and the telephone will ring while the calling party will hear a ringing feedback tone at the other end. When the called party picks up the telephone, both the ringing and the associated ringing feedback tone at the other end will stop as one would expect. However, the calling party will not hear the called party (one way audio) and the called party may not hear the calling party either (no audio).
"The issue of NAT Traversal is a major problem for the widespread deployment of VOIP. Yet, the issue is non-trivial and there are no simple solutions."