I am not totally familiar with the 2600 series, but I do know you need DSPs. I am not sure if you have any (do a sh ver and it should say something about DSPs being installed if any are detected. You must have IP Plus or better feature set for this). Withou DSPs, the router cannot convert the signal.
I would then suggest you break this down into part.
Step 1)
Get the T-1 signalling working between the 2600 and the PBX.
Step 2)
Set up your dial peers for the VoIP provider.
Have you found a VoIP provider? There is a lot to consider when using VoIP outside your LAN. Many things can go wrong, and many things can effect the quality of the voice. Most providers have not developed a solution for this (except cable companies because it never leaves thier LAN) because too much can go wrong and they will loose business.
We have Net2Phone come in 3 years ago and try this with us. The refused to use a Cisco solution and believe me, it never worked. I did, however, get a nice 3640 router out of the deal and a PRI card for my PBX.
Do some searching on Cisco's website.
These explain how to hook up the 2621 to different PBXs. Don't worry about the call manager stuff, the router configs should be basically the same for the serial interfaces. The only thing that will be different is the dial peers.
It is what it is!!
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A+, Net+, I-Net+, Certified Web Master, MCP, MCSA, MCSE, CCNA, CCDA, and few others (I got bored one day)