By Avaya Definity system, I'm assuming you mean Avaya's ECLIPS Enterprise IP Telephony system, as opposed to the generic circuit-switched Definity.
Normally, IP Phones talk a protocol called SSP (often referred to as simply "skinny") on a client-server basis to Cisco's CallManager. SSP is open, but vendor-specific (that's a marketing term for proprietary).
In theory if ECLIPS can support SIP, and you use the SIP code on the IP Phones, it SHOULD work as SIP is a "standard", but no other way. Personally i'd want to spend time testing it first, but no doubt Cisco have got info on some customers or tests where they've seen it working.
Hope that helps.
