CDMA (Code Division Multiple Access) is a "spread spectrum" for cellular networks enabling many more wireless users to share airwaves than alternative technologies.
GSM (Global System for Mobile Communications) is a wireless technology to describe protocols for cellular networks used by mobile phones with over 80% market share globally.
LTE (Long Term Evolution) is a wireless broadband technology for communication of high speed data for mobile phones.
The main difference between the technologies is that both CDMA & GSM can support cellular and data, whereas LTE can only support data.
So what does this mean? It means that if the phone does not have CDMA/GSM technology then the cellular (voice) services would NOT work, only data.
Prior to the entrance of LTE Technology, it was very easy to determine a CDMA phone from a GSM phone. The GSM phone featured a SIM Card Slot, the CDMA phone did not. Now it is quite more complicated with the LTE Technology, both CDMA & GSM adopted this spectrum in which functions with the use of a SIM Card. Therefore, a CDMA/LTE Verizon Wireless phone now has a SIM Card but does not work on GSM Networks such as AT&T.
There is two main reasons why CDMA/LTE and GSM/LTE are currently NOT compatible;
1) Phone (voice) runs on the 2G & 3G spectrum. CDMA and GSM run on different 2G/3G frequencies, thus voice is not compatible between the two.
2) LTE (Data) the common denominator, runs on the 4G spectrum. CDMA currently runs on Class 13 and GSM Class 17 LTE, thus data is not compatible between the two.
What LTE phones can be unlocked for GSM usage? In order for an LTE device to be able to be unlocked, it must have GSM technology so cellular and data will function.
In the below LTE phone examples, the AT&T Atrix HD can be unlocked because it has GSM capabilities, but the Sprint Evo and Verizon Thunderbolt can not as it only has CDMA technology.
AT&T Motorola Atrix HD (MB886) - (2G) GSM 850 / 900 / 1800 / 1900 - (3G) HSDPA 850 / 1700 / 1900 / 2100 - (4G) LTE 700 MHz Class 17 / 2100
Sprint HTC Evo 4G - (2G) CDMA 800 / 1900 - (3G) CDMA2000 1xEV-DO - (4G) LTE 800 / 1900
Verizon HTC Thunderbolt - (2G) CDMA 850 / 1900 - (3G) CDMA2000 1xEV-DO - (4G) LTE 700 MHz Class 13
To complicate things a little more, the below example supports CDMA/GSM/LTE. If this phone were unlocked, it would only run GSM on the 2G spectrum without 4G LTE. (Verizon uses a separate 4G spectrum than other GSM networks.)
Verizon Samsung Galaxy S4 - (2G) GSM 850 / 900 / 1800 / 1900 (2G) CDMA 850 / 1900 - (3G) CDMA2000 1xEV-DO - (4G) LTE 700 MHz Class 13