It depends on what service provider/carrier they are using. Each one uses either GSM or CDMA networks. A few, since they don't have their own networks, use other providers networks. So if the provider uses AT&Ts network in one area but then uses Sprints in another, they would be GSM in the one area and CDMA in the other. Phones are one or the other. So you need to start with what network they will be on (carrier/service provider) and then get a device that will work with them.
CDMA Carriers in the USA ...
http
/www.tomsguide.com/faq/id-3143114/cdma-carriers-usa.html
GSM Carriers in the USA ...
http
/www.tomsguide.com/faq/id-3143113/gsm-carriers-usa.html
Also, please note, before getting a device, be sure to check directly with...