Interoperability is a sore spot among some VoIP admins because of the multitude of signaling standards, RFC revisions, and competing commercial standards. Interop problems are the main reason lots of enterprise decision- makers used to talk down VoIP technology and perhaps the key reason VoIP wasn't a rising star of enterprise telecom sooner.
Of course, talking about interoperability problems is easier than pinpointing them in the field. During a signaling exchange, two participating endpoints may not be "on the same page," so to speak. Caller and receiver may be assuming a different approach because, though they're both using SIP, there's enough leeway in the spec to allow for app-specific "dialects."
Mismatched capabilities is one example. A video phone might have a preference for video calls, but it could also be configured to permit only video calls. Audio-only calls could be crippled due to a configuration mistake. Or one manufacturer's preferred authentication scheme might not work very well on another vendor's PBX. It's also possible that a particular user agent may not fully implement the protocola softPBX that doesn't support SIP redirects, for example.
There are a lot of interoperability pitfalls here. The test lab is the best place to discover and isolate problems, before that shiny new equipment's in the field. Not all problems can be solved in the lab, so interop troubleshooting is something you'll soon be doing, if you aren't already. To troubleshoot VoIP issues, put yourself in the endpoint agent's situation. Find out what the endpoint is seeing and how it is reacting . Do the same for the server. The key to troubleshooting interoperability between two VoIP agents is asking, "What does this device think it's doing?"
Looking at captured packets will often tell you what signals a device is sending, but not why it's sending them. For that, you'll need to go to the device's logfiles and diagnostic data. Comparing the caller's account of the interop event with the receiver's account will help you reveal the root causes of interop problems.