The best (and unfortunately, most ridiculously expensive) one I found was by Motorola. I used it for a Limo service that ran from the airport to some of our downtown hotels. IIRC, it had five-ish presets for speed dials (dialing while driving is scary for passengers) and had a 12-button keypad. Each of the three hotels that was using the system had a button, and the drivers could dial regular numbers from their keypad.
It was 2-meter, so it had pretty good reach and didn’t require a lot of support (we had one tower on top of the tallest hotel).
Now, having said that, if I was to do it again, I wouldn’t have gone ROIP (the hotels were insistent, but I think it was more of PITA than was needed).
Using a cell phone (simple flip phones work, call them “communicators”) and a DISA call-back connection, you can get dial-tone from the system, be on the local machine, and have the level of security maintenance guys need while monitoring their activity. Your phone book is then the one from the system (instead of having to do downloads) and you get the full features of the system available.
If your WiFi is really ubiquitous, you could also go with a hand-held (vice base-station wireless) phone. A DECT network (once they get roaming between stations working well) could do what you are looking for. Like you said, getting coverage at the edges becomes a problem, but Tranzeo sells some good equipment for the corners (wireless hot spots combines with long-reach PTP radio systems) and for ‘remote’ campus systems.
A couple of ideas.