We traveled 46 states with AT&T and Verizon. Rarely found a spot that they did not work.
We switched from former to latter when they could not tell us if our phones could tether our laptop, after several frustrating phone calls and a visit to an AT&T office. Verizon verified that the phones could not tether, showed us new phones, demonstrated tethering, killed the old phones, ported the new phones and had us headed home in 1/2 hour.
Matt B