Radio spectrum is "owned" by the federal government and leased or sold to private entities at auction. While you can use ISM/UNII bands ("free"/open) spectrum (and there are GSM base stations that run in 900Mhz), you have to deal with interference from every other device in the band. Cell systems are designed to generally not interfere with each other through RF planning and synchronization of TX/RX timeslots. They don't work well in the face of outside noise.

Now, in areas where cell providers don't see ROI on a physical build they will sublease spectrum to a local operator in exchange for roaming rights (sometimes transparently--you might not see "roaming" on your phone). Verizon and AT&T do this in chunks of the midwest.