A cellphone must be deactivated by its associated service provider or its SIM card must be removed to prepare it for resale. The method required depends both on the type of cellphone and the carrier associated with the phone.
There are two primary cellular network technologies, CDMA and GSM, in use by cellular providers as of April 2015. While there are numerous technical differences between these technologies, one of the major practical differences for consumers is that CDMA phones are internally locked to the cellular network of their associated cellular provider and must be deactivated or unlocked by that provider's customer service department.
In contrast, GSM phones store cellular provider information on a removable SIM card, meaning that the phone is effectively deactivated once that card has been removed. Some CDMA phones also have SIM cards as well, either since the phone was designed for use in foreign countries where GSM is the dominant technology or for compatibility with newer 4G cellular networks that are GSM-based. These phones still need to be deactivated by the carrier itself due to their CDMA-compatible components.
In addition to deactivating a phone before resale, it is advisable to perform a factory reset on the phone to remove any personal data. Some manufacturers also provide additional services that are registered to a phone separately from the cellular provider, such as the iCloud services provided by Apple. It is also necessary to disassociate a cellphone from any such services before reselling it.