compatibility issues

In brief, I am looking to buy a device, which can send SMS to a person's cell phone from a microcontroller.

I found a device Arduino GSM Shield, which can send SMS from microcontroller but I am not sure if Arduino GSM Shield is compatible with the technologies of our local service providers , like AT&T or T-mobile, as it does not use EDGE or UMTS technologies to send data.

The link for the list of cell phone service providers technologies is http://en.wikipedia.org/wiki/List_of_United_States_wireless_communications_service_providers

The link for the list of cell phone service providers frequencies is http://en.wikipedia.org/wiki/Cellular_frequencies

Please confirm me if I will be able to use Arduino GSM Shield to work with AT&T or T-mobile Please reply as son as possible