I have a Cisco 7942G IP phone running SIP firmware version SIP42.8-3-2S. I have been trying to get the phone to register to various 3rd party SIP telephony service providers, and I keep running into a wall with NAT issues. In simple terms, if I connect the phone directly to an internet gateway (i.e. modem) the phone registers fine, but it fails to register as soon as I put it behind a router.
I did a debug sip-messages and I was able to see that the phone sends SIP REGISTER messages to the SIP provider, but there seems to be no reply from the provider. Hence, I conclude that the SIP replies are not being routed back to the phone, and that is why the phone is not registering.
Now, this phone is under a multi-tenancy environment, which means that I have no direct control of the router, and thus I cannot port-forward or create static routing rules. The interesting thing is that the SPA series phones and the WIP310 work perfectly in this environment, but it seems like the 7900 series phones are extremely limited when it comes to SIP.
Do you have any recommendations on how to make this phone register considering the environment in which it is in? This has become a NAT nightmare for me. I have attached a copy of the SEPxxx.cnf.xml file.
There is quite a lot of SIP NAT stuff in most phones. On my 7961 ( yes its old!! ) Under Settings -> Device Config -> SIP Config -> SIP General Config ... I have settings for NAT Enable. IIRC there used to stuff in there for STUN too, but that was many years go.
The only NAT option on the phone is true/false and then you need to specify xxx.xxx.xxx.xxx
This is not helpful because as I mentioned above, it is a multi-tenancy enviornment, and I have no control of the router. I can't port forward SIP to the phone because I would disrupt everyone else in the environment.
Also, my understanding is that the 7900 series do not support STUN.
How is it possible that my SPA500 series and WIP310 register and work with no problems behind this router. I'm wondering if there are NAT-transversal features on these newer phones that the 7900 series doesn't have?
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