We started using 3CX in our office in the past week in the hopes of replacing our old system; at the moment we have it mostly working, but we're having difficulty with two of our phones. These phones are outside our network behind another network's firewall. (This is a m0n0wall system on both their and our end.) They are registering just fine with the PBX box and calls can be made from them to the other extensions, but not the other way around. Every attempt thereof provides us with a 408: Request Timeout error. All ports are open to the two phones on the opposing firewall, and we have tested with all ports open to the 3CX box through our firewall; this has not solved the problem. We've been able to connect another external phone to our system that is also behind a firewall with no problems, though it is a very different model; the two phones in question are Grandstream BT-200 models. As all IP addresses involved are static and communicated to the outside world with properly-mapped ports, no STUN servers are being used; everything has been manually entered. When we tried using STUN servers, everything behaved in the exact same manner. The phones can very easily be accessed via HTML from our end, so it's not a sudden loss of communication either. We've tried providing audio through the PBX, but this doesn't change the situation either. Any tips in the correct direction would be greatly appreciated.