Asterisk to Avaya IP400 Link

I’m just starting to play with Asterisk (hoping to use it in a forthcoming new building) and I was wondering how I can go about linking it into our existing Avaya IP400 system.
Initially we are looking to run them side by side (the Avaya system linked to our three ISDN2E boxes) with the Asterisk system providing all the new phones soft and hard) and Avaya the existing mix of analogue and digital phones. In the long run the Avaya system may be taken out.
I have seen a few mentions of people trying to do this, but with not a lot of success. So, can anyone give me the following:

  1. The hardware needed by both the Avaya and Asterisk systems to provide the link
  2. The configuration of both sides to provide the link.

Not asking for much I know :wink:

Help much appreciated