You need to contact the service providers to see which unit is on what network, because a lot of these units use the GSM or SMS bands, so technically networks don't make as much as a difference as they used to, since GSM is GSM, no matter the provider, it's just who they have agreements with.
You could also try Connect24 (DSC) they tended to have coverage in areas where our Telular units didn't have enough signal.