well, that's my thought as well, but it's no different being in between say Verizon and our phone tech. Phone tech insists it's Verizon (which it usually is) and Verizon saying it's the switch. Progress is halted and both parties give up.
I'm not entirely sure it's Call Accounting tey're talking about. I made sure PBX was talking to CA myself with the CA vendor and some tweaking. I think they're actually referring more to the Hotel/Motel functionality in the switch Check In/Check Out, Clean/Dirty etc.
From what I understand the will communicate with the PMS in the NEAX Model 90 protocol, but doesn't do so natively. I'm curious what program controls how the 8100 speaks to the PMS. There's nothing in 42-xx except port assignment etc.