Asterisk to Avaya 8300 via E1 connection

My customer has an Avaya 8300 and wants to connect an Asterisk server with this system via an existing E1 connection. The Asterisk server will be used to feed about 25 IP phones as a standby Call Centre system should the Avaya fail but will also need to be able to communicate with the Avaya system extensions (and Avaya to Asterisk). My only real concern is geting CLI across the E1 circuits. Does anyone have any experience of this sort of connection to such an Avaya system?