This has strayed pretty far from the original topic. The original poster asked a question about using a shared system for limited calling. We do that in hotels and motels all the time. A basic Mitel system, like maybe an SX-200 analog would pretty much fill the bill as it can trap and report any 911 call in real time.
I wouldn't screw with incoming service as it drives the initial cost and monthy recurring charg up quickly. An SX-200 Analog, carded 4x32 with console and a Viking apartment entry system ought to be available for less than $4K with a monthly cost of about %60 for 2 POTS lines.
Just my $.02
Carl Navarro