VOIP over 3G network

Question

I have created a STUN-like + rendez-vous server. I have tried the whole thing over WIFI (behind a domestic NAT), and everything works fine. I have two mobile ISPs, one allowing everything (VOIP + P2P + MODEM) (ISP 1) another one allowing everything except P2P (ISP 2).

When trying over the ISP 1, it works fine also. However, when I try with the ISP 2, udp packet doesn't go through.

I have connected my computer with the phone on ISP 2, and ran TUM NAT Analyzer.

It is telling me

UPnP Test (?):  No UPnP device found

STUN Test (?):  Symmetric NAT

UDP Binding Test (?):   Endpoint depenent binding, port prediction may be hard
TCP Binding Test:   Endpoint depenent binding, port prediction may be hard

UDP Mapping Test (?):   local and external IP addresses were different 
(NAT). Your source ports were not preserved. It may be hard to predict your external source port.
TCP Mapping Test:   local and external IP addresses were different (NAT).
Your source ports were not preserved. It may be hard to predict your external source port.

SIP ALG (?):    The initial SIP INVITE packet has not been modified on its way to our servers.
There is no SIP ALG involved
FTP ALG:    The initial FTP PORT command has been modified.
Most probably, your NAT implements a FTP-ALG

So apparently it is using a symmetric NAT with random port affectation (no way to use port prediction apparently).

So I was wondering, an ISP that allows VOIP but not P2P (And no SIP ALG), does it expect VOIP to use a relay server in order to work ?

Or am I missing something...? As I understand AT&T (and probably others) uses the same kind of NAT as my ISP 2... (symmetric NAT) so that become a huge issue, I guess....

Anyone though, idea, reaction would be great.

Answer 1

• Lot of VoIP providers include infrastructure based on SBCs or other RTP proxies that in any case will relay the media traffic no mather the type of NAT you have.
• If the other endpoint is not behind a symmetric NAT (for example a PSTN gateway that is a very common case) you can connect to them without any additional relay.
• Otherwise you need a TURN relay