Additional reading material:
The government determines which bandwidth within the spectrum that can be used for wireless communications and sell (or auction) space to the Service providers. Once they have their piece of the spectrum the Service providers supply the network (or “highway”) and the handset manufacturers provide the mobile devices (or “cars”) that travel on it. There are two main types of network technologies for wireless communication (CDMA or GSM), either one does a fantastic job at transporting your voice and data streams up to certain speeds.
“4G LTE” is a faster network for data signals as it operates at a bandwidth that’s higher on the electromagnetic spectrum and is independent of the voice network, so all service providers provide 4G LTE service regardless if their voice network uses CDMA or GSM.
Some of the main differences b/w CDMA and GSM relevant to the original question:
Global System for Mobile Communications or “GSM” networks use a SIM card to store subscriber information so you can move your SIM card from one GSM handset to another GSM handset and the network doesn’t care (all is good). This type of network is predominant in Asia and Europe, and used in the USA by companies like AT&T, & T-Mobile. Therefore depending on how you want to use your phone when travelling, get a local SIM, slap it in your device to avoid roaming charges … if you want to stay in touch with people back home then use your USA SIM (that’s why there are dual SIM devices now).
Code Division Multiple Access or “CDMA” networks keep subscriber information safe in the CORE of their network. Combined with their inherent encryption algorithms, CDMA networks make it almost impossible for anyone to “clone” your phone … that’s why the US Military developed and use this technology as well as USA companies like Verizon, Sprint, and Cricket.
Handset manufacturers like Samsung for example, will make a device with its own operating system (Android, Windows, etc.) then add all kinds of features, bells, & whistles, but they still have to decide which wireless technology to put in the handset. Therefore they often make a device that works on GSM networks and an identical sister model that works on CDMA networks. Each of these phones can be equipped to operate within a given spectrum or spectrums. Not all governments allow wireless communications within the same area of the Electromagnetic Spectrum, so each of the phones may be offered with various combinations of spectrum capabilities (“Global phones” offer the biggest number of combinations). Now you understand why there are so many model numbers for phones that physically look to be the same … but they are not.
For those who say Verizon offers “some” GSM devices … that is true. Sometimes a CDMA or GSM provider will purchase a smaller competitor who has a network using the other technology in a specific area. That’s where the “roaming wars” begin for the providers, but not a concern for subscribers as your device will work just fine regardless of who gets your money.