MVNO

A Mobile Virtual Network Operator is a wireless communications services provider that does not own the wireless network infrastructure over which it provides services to its customers. An MVNO enters into a business agreement with a mobile network operator to obtain bulk access to network services at wholesale rates, then sets retail prices independently.[1] An MVNO may use its own customer service, billing support systems, marketing, and sales personnel, or it could employ the services of a mobile virtual network enabler (MVNE). http://en.wikipedia.org/wiki/MVNO

company that pretends to be a CellCo but really just licenses services delivered by another company.

Virgin Mobile is an MVNO.

US List: https://en.wikipedia.org/wiki/List_of_United_States_mobile_virtual_network_operators