The (US) National Commuications System posted:

During emergencies, wireless service providers can experience congestion in their networks. Such congestion can severely curtail the ability of personnel with National Security/Emergency Preparedness (NS/EP) responsibilities to use wireless services. To facilitate the completion of critical calls during these high usage events, WPS [Wireless Priority Service] enables NS/EP personnel to access the next available wireless channel before subscribers who are not engaged in NS/EP functions. Additionally, priority calls will not preempt calls in progress nor will WPS users monopolize all available wireless resources.

Carrier participation in the WPS program is voluntary under the FCC Report and Order. To encourage participation, the National Communications System (NCS) is funding the technical development and implementation of priority features in the carrier networks. Although the FCC maintains oversight responsibilities for the WPS Program, the NCS manages the day-to-day administration on behalf of the Executive Office of the President.

When an approved WPS user wishes to change service providers, whether transferring their existing number or obtaining a new number, they must have their organizational WPS Point of Contact submit this change request online. The NCS will coordinate between the two service providers to transfer the WPS feature, if WPS is available in the service area of the new service provider. The new carrier cannot accept a request from the user directly to add WPS to the user’s new account.