Hi dunce
i have been working on Avaya platform for long now and i do not see the possibility of avaya phones working behind cisco the reason being Avaya phones run the DCP protocol proprietary to avaya on the top of the standard h.323 protocol. so the avaya phones will not get registered on the call manager. May be if u have a network analyser u can see the protocols and get a better idea.
regards,
vivek