Most Elavtors i've worked with have an analogue dialler in them,
Pretty much an analogue extension, I have actually installed a Panasonic NCP500 to service a hospitals lifts before and it works fantastic.
You pretty much need a pair from the lift control panel to your PBX and pin it out as an Analogue phone,
I think the 824's are a bit more limited on the hotline side of things, maybe setting up Pickup Dialling would work,
I have attached a page of the 824 manual which should help you, you may need an SLT handset to do the initial setup
Calum M
ACSS