The damn things have the WORST USER INTERFACE IN THE WORLD.
Do you have any idea how to dial back a missed call on 5224 IP phone? It's a fucking nightmare! Here's the thing: unless you have done the right magic invocation, you can't. Nope. It's an "advanced feature" or something, somehow, so you have to program a programmable key to invoke the advanced function, and then you have the pleasure to enjoy the retarded user interface that the sick, sick, sick minds at Mitel have produced.
And forget about those calls you missed before doing the hocus pocus, it's lost forever.
What are the invocations, you may wonder? Well, let me put it this way: there is absolutely NO WAY you can find out just by using the phone. It's completely random. In fact, I wouldn't be able to do it again.
Are there people working on UI at MITEL? Rhetorical question ...