Oh man, I completely forgot about that O/P port because I never have to use it - thanks!
So if I'm understanding correctly, could you take a 3.5mm audio cable, plug one end into the O/P port, then cut the jack off the other end and splice it onto the cable that was running into the old contact closure from the door phones? Or is it more involved than that? Then just configure a button on the phones to dial the *39/*42 Relay 1/2 short codes when they want to open the doors?
ETA: Searching on this site finds conflicting information, with many recommending connecting the O/P port to another relay (like a Viking RC-2A), and then to the contact closure wiring. Normally we'd just use a Viking C-2000B and Avaya door phones, but this customer wants to reuse their existing door phones and to be able to open the doors with the press of a button like on their old ACS. The problem is they're not sure who handles their door headers (Stanley, etc), so we don't have any way of finding out what wiring is in the header. Is there a way to use the C-2000B and connect it to the wiring from the Partner Contact Closure and have it do the trick if the O/P port solution isn't an option?