My original post seems to have disappeared, so I'll repost...

Are the Avaya 4610/5610SW phones compatible with any other systems? We own 20-30 of them.

We're migrating off Iwatsu to a VoIP, but we're not too keen to try and implement IP Office V2 as it's older than the Iwatsu we're leaving.

IP Office V2 is old? Unless you mean 406V2, this is Avaya's flagship SMB product where all R&D money is going. To answer your question, 4610 work with both IP Office and Communication Manager, while 5610 work only with IP Office.
Mike

Ugh. Yes IP406.

So buy a IP500V2. If you already own the phones, you're still way ahead.
Mike