They are on sprints towers currently (like, well every other MVNO in the US) there is a rumor I heard that they are trying to partner with Verizon as well (those being the two major CDMA providers) which would pretty much kick ass. lol.
The carrier is of course them selves. Helio. thats it. Its like Boost Mobile or AMPed. They provide the rate plans and the phones the actual service is based off the larger networks towers (and with sprint being basically a network whore) you see it almost always being sprints towers (probably because their plans suck BURN. jk)