I just went through this last summer. The client my company works for replaced all of the Avaya Phones with Cisco 7960's, running on a Call Manager version 4.2 cluster. They brought in a team of consultants to do all the programming and deployment.
If I had not been watching over them every step of the way, they would have missed all of our VDN's, Vectors and hunt groups. I asked them why these numbers (VDN's) weren't on the cut list, and they said there were no stations on my switch programmed with those numbers. To make my point, I had them dial one - used for password protecting a bridge line, to keep automated spam/telemarketer calls from getting through to the bridge. They asked how we were doing this. When I said VDN and Vector, they said "What's a VDN and Vector?"
Things Cisco can not do: Time of day routing; Holiday routing; Audio Streaming. We had to beg to get department hotlines that rang on everyone's phone, and had to have the "c-barge" feature turned on so that more than one person could be on a "shared line" simultaneously.
We are still using our Avaya switch to provide streaming audio of a couple of cable news channels. People with Cisco phones dial a number that stayed on the Avaya and listen through the speaker. Cisco simply can not do a live audio stream. Only audio files.
Know what you have, document everything, and watch them closely (assuming someone else is doing the conversion). Get training on the Cisco platform BEFORE converting. Or else on day one you'll be scratching your head trying to figure out how to fix the problems. And there will be problems.
There is absolutely nothing about my Cisco 7960 that is better than my Avaya 2420 or my 8434DX (which is still running on my desk, as it's MINE, not the company's, and will be running until they come in and unplug my Avaya switch).
- Duaneness