I concur that Allegiance uses Sprint for their communications. Be aware that they recently bought the DFW operations (and some other areas) of Lone Star and Guardian so there may be some consolidation and migration of those company's operations to match how Allegiance works.
To address the "Nextel shut down long ago" issue, be aware that the old Nextel push-to-talk system isn't the only thing that Sprint offers to businesses for their communications needs. I believe that this (https://www.sprint.com/business/solu...l#.U-WXtU10yUk
) is what is being used today when folks say the EMS companies are using the Sprint or Nextel for their communications.