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.
While both companies have H.323 phones they both use underlining proprietary protocols within the system architecture. For Avaya it is CCMS. There may however come a day where this won't matter due to SIP.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.