There are many employees that have many different carriers. Our companies corporate plan is on ATT and everyone with their service has good reception. All other carriers get poor reception.
As your corporate plan is with AT&T, what probably is happening is that your company has a network extender for AT&T in the building. If having reception inside your building is a priority then you should look at AT&T as a provider. My info is that they have good reception in the Twin Cities metro area but reception falls off in some of the 'outstate' areas.