You should know that 99% of all cellphones sold in the US and Canada are compatible with mutually exclusive groups of network carriers.  What this means is that some carriers use certain communication-technologies and will not "talk" to devices of competing technologies.  The two primary digital technologies in the US and Canada are GSM and CDMA.  There are a few older digital technologies that are still being supported to some extent by the carriers, but typically they are only on a "grandfathered" basis.  Those older digital technologies are called D-AMPS and TDMA.  Explanation of the differences between the technologies is outside the scope of this guide.  All carriers support either CDMA or GSM, yet those who support one, do *not* support the other.  Therefore, if your carrier is a GSM system, you will not be able to activate or use a CDMA device (phone, PIM, etc...) on their network.  The reverse is also true for CDMA systems.  Be careful on eBay with bidding on phones.  I have seen some auctions as recently as today, where conflicting networks were listed as compatible with the phone they were selling.  I do not know if the seller is hoping to gain something, or is just ignorant as to which networks use which phones.  When you buy a device to use on a CDMA network, make sure the auction lists it as a CDMA device.  The device manufacturer is inconsequential, as most makers will create phones for both network-types.  An example of this is the Motorola RAZR.  The GSM-compatible version of this phone has a model-name of "V3", while a CDMA version of this phone is called the "V3C".  Even the Firefly from fireflymobile.com will be shipping a CDMA version of their populat GSM childrens' phone in the near future.

CDMA carriers (in no particular order):

US Cellular, Verizon, Alltel, Sprint, Telus, (and some other more obscure companies)

GSM carriers (also in no particular order):

Cingular, AT&T, T-Mobile, Rogers, (and other smaller companies)