Digital cellular service uses several competing standards.
In Europe and much of the rest of the world outside the United States, the standard is Global System for Mobile Communications (GSM).
GSM’s strength is its international roaming capability.
There are GSM cell phone systems in the United States, including T-Mobile and AT&T.
The major standard in USA is Code Division Multiple Access (CDMA), which is the system used by Verizon and Sprint.
It transmits over several frequencies, occupies the entire spectrum, and randomly assigns users to a range of frequencies over time.
In general, CDMA is cheaper to implement, is more efficient in its use of spectrum, and provides higher quality amount of voice and data than GSM.