Former user here with VZW for most of 25 years, going back to the NYNEX mobile & alpha-numeric/tone-only paging days, also had AT&T WS/M for a # of years as secondary before switching some lines over to T-Mo. From down south along FL & up/down the East coast, unless your corporate team are heading into the woods, coverage should be almost as good & nearly as good - depending on budget & hardwares, choose your mobile smartphones carefully for best use of the 4G/LTE networks & towers - for optimal integration of cellular, WiFi and roaming/oversea coverage on all the single mobile # including texting & data access.
It sounded like your office folks just wanted nothing more than the ability to flip the switch, get the virtual dial tone (wireless signal) and able to make/receive calls - and not worry about anything else & hate reading/following instructions and prompts.
T-Mo has a business division and reps that can work with you (deals for up to 10 lines, credits/rebates for porting/moving lines & even dealing with ETF's and so forth) There is also a money-back/worry-free program to try out for 30+ days (??) as I was just reading it on my (formerly Sprint) iPad mini this week now on T-Mo's free monthly 200MB FDFL program - useful for an upcoming trip as it has the global LTE bands abroad.
Simple Choice North America Plan would extend coverage to Canada & Mexico, recently rolled out - and, without contracts, as others suggested, get a new line for 30 days and for as little as $70 or $80, the user will get unlimited calling & texting, and as much as 3 to 5 GB fast 4G LTE data to use - (speed is faster than/comparable to AT&T's signal/coverage) Aside from iPhone 5s or 6 - not locked to any carrier, of course; devices like the Nexus 6 or Moto X Pure Ed. or 3rd. generation "G" might just be the hardware ticket to convince the majority to take the plunge.
Home in metro NY, VZW continued (no plans to add extra towers for the fringe section of our "town" ... just a few dead zones to them, hmmm) to the worst, and surprisingly - indoor coverage with T-Mobile is even better/stronger reception over AT&T. Out on the West Coast, T-Mobile is just fine too ... except inside the Grand Canyon NP, while vacationing. On the whole, we switched 2 years ago & not missing Big Red at all (for those who hasn't tried T-Mo in the last year or two, I'll say give them another run for your money - there's a $30 prepaid plan for 5GB data, unlimited SMS & 100 mobile minutes, and, the 3-in-1 Sim card is (was) free again.
Where are their overseas trips landing them - FT's are reporting good success with the free roaming data all over under T-Mo and not paying extra, try doing that under the "Can you hear me now" banner
