I'm a 3CX partner, and am trying to do a demo for a client. I've got the demo 3CX version 12.5 running on a server in my datacenter. Static IP with 1:1 NAT on the Sophos UTM, ports 80, 443, 5060-5061, 5060 UDP, 5000-5001, and 9000-9199 open inbound. At the client site, they've got AT&T DSL (blech) with their modem/router and a Netgear FVS-318G behind it. I've got two Cisco SPA504G phones there. At first, I got one phone to register but the other wouldn't. They both showed as registered (green button), but 3CX showed only one as registered and you'd get a "not found" calling that extension. Fiddled around a bit, rebooted everything, and after a while, the other one worked but the first one didn't. A couple of days later, both were connected just fine, I could call both, etc. Today - another couple of days later - neither would register. I tried just about everything. The phones are set to use stun.3cx.com, and then I tried stun2.3cx.com. BTW, I manually configured them (not provisioned from a template). They're set up almost exactly the same as my SPA962 on my own desk which connects just fine (I set up an extension to their 3CX). I also have the 3CX v12 phone app on my iPhone 6 configured as an extension (not using the tunnel). When I connect from within my office, it works fine. When I use Verizon LTE, it works fine. When I use the client's WiFi, it doesn't work. Anyone got any ideas? I'm very frustrated, and the fact that I can't get the demo to work in the client's own office makes me look like an idiot - and doesn't bode well for trying to sell him 3CX (a pretty decent sized system with 20 remote reps and three separate offices).