Sorry no experience Migrating phone systems.
I've been around several phone systems, doing tech support & consulting. I've been using Mitel for about a year, and seems ok, a bit different terminology than Nortel and Avaya.
Flexable paths, ques, or hunt groups. Good smdr reporting, alerts, and notifications, also has unified messaging with voice messages to email.
Diagnositics and trouble shooting tools could use some improvment. As a call center with around 200 phone reps, over voip, we get the information we need from the system.
Sorry I couldn't assist with any migrations from IPOffice, good luck.