Hi Hydro !
GSM uses a different cell signal than the CDMA. MOST of the world uses GMA. Only Verizon and Sprint use the cdma.
SO, if you get a new phone for example, if you want it to be compatible with ANY provider get one that is compatible with both.
I use strictly GSM because I have mnore choices and I do not care for Verizon or sprint.
Personal matter and choice.
That is why the phone one gets needs to have the correct hardware to access either of the two signal bands or if you use only one of the providers get the one that is right for your preference.
MY Phone will NOT work with Verizon.