Yesterday I spent quite a bit of time helping one of our school district troubleshoot an IP call. Here are some of the things we tried.
Check the IP Address
One of the first things to check is the IP address that the other location gave you to dial. In this case, the first number they gave us was a 10.10.xxx.xxx number. Any number that starts with a 10 or 172 is an internal IP address and so you know up front that you can't dial that number if you are not on their network. When this happens, ask the other location to give you an "external IP address" to dial.
How have other connections worked?
The next investigation is how can both sides connect? Ask yourself and the other location – where else can I connect that it works? Can I connect with other places outside my network? Has it been tested recently with another location? In sorting through this conversation, we discovered that both locations could only place IP calls. Neither could receive. So then we set up a connection on my bridge so the two places could call in and meet each other there.
Next we discovered that we were getting video & audio from the other location, but they couldn't see or hear us. They were seeing a black screen. This is a red flag for firewall problems. It's VERY common (at least in my experience) to have the connection work almost all the way, but one or two parts are missing. Often the firewall lets the call connect, but doesn't allow the incoming audio & video. At this point you ask the other site to check the ports on their firewall or ask them if they've successfully connected to someone else outside their network.
On this particular call, we gave up on IP and went to ISDN. That was as much of a challenge as they gave us three numbers for a switched call. We ended up making it work with a 2×64 call and I bridged it from myPolycom VSX 7000. Thankfully the connection went well.