I'm contemplating adding a "call-home" function to a mobile embedded system. The only data to be sent is a set of GPS coordinates and eleven bytes of status data. The easiest way I can think of doing this is to put a GSM phone in the thing and send an SMS. Can someone recommend a GSM1900 phone readily available on eBay that has an easily-interfaced, well-documented serial port, and that can be used with a cheap prepaid service that supports SMS?
Or perhaps someone can suggest a different option? What other radio networks exist that cover the NY metro area and are cheap to access infrequently (I don't want to pay a $50/month service charge)?