For the record, I’m a big fan of IP based phone systems for a whole bunch of reasons I won’t cover at the moment.
What I do take issue with, and what you need to watch for, is the split in service delivery. “Back in the day” the Telco, BC Tel around these parts, of course, did it all…Phones, Dial tone, Long Distance, Wiring. Not advocating a return… WAY expensive, few choices.
BUT today, it’s quite common to find companies that have a different vendor for each, heck could be multiple vendors. Here’s an alert… good wiring is important. Unfortunately, what once was regarded as a craft has become a commodity.
A professional installation includes clear, neat labeling at the wiring blocks, labels at the jack, and a matching floor plan. Make sure you have voice and data jacks everywhere… spec the data jacks as EIA 568B minimum, http://en.wikipedia.org/wiki/TIA/EIA-568, although we’re past that with “Cat 6″.
Two more tips…watch out for cheap data wire, spec a name brand like Pirelli, Belden, Infinity, Essex, General cable. There are low grade clones out there that don’t meet the performance specs.
Finally, ask for pictures from previous installs… people who take wiring seriously will have pictures of their work. I do.