I just spent some time banging my head against the wall trying to figure out why my Snom 370 could receive calls from my Elastix PBX, but could not make outgoing calls. I kept getting a “not allowed here” or “not found” when dialing other extensions on the PBX.
The answer is to disable two key settings on the extension identity tabs. Both are found in the RTP tab, and on my 370 only the first was already turned on. The settings to turn off are RTP Encryption and Symmetrical RTP. Here’s a nice screen shot with the settings highlighted.
Now I can call other extensions like normal.