You could say that all Cell phone companies like ATT, Verizon, etc, are in the cell phone SELLING business. That is, they most likely make more money selling you there exclusive phones then they do selling the service. What companies like ATT, Verizon, VM do is go to companies who make cell phones and have them make phones that work only with their servers. That way, you can't use one phone that works with Sprint to work on VM or any other company. Some phones work with a SIM card, others don't. Operating systems and formatting all play a part as well. It all has to do with competition and maintaining exclusivity.
answered 3 years, 1 month ago