Avaya uses two types af digital handsets, the Digital Terminal, usually referred to as 20XX ad the Digital Stations, that is the 24xx, 44xx and the 64xx range.
Both types uses different type os line cards and are not compatible.
About the function you would like, Avaya calls it "Force Feed" on the INDeX platform, taht's were the DT range is coming from. I don't know if this is possible at all on IP Office, at least i could not find a way to do it.