Hi Al,
I did something like this three years ago, using a Multi-Tech modem for CDMA networks. (CDMA is popular here in the US, most of Europe is GSM.) I allocated an array with ten bytes, three for the area code, and seven for the mobile telephone number. On our network you don't have to dial a 1 first, but other networks might require it. When a user sent an SMS message in to the system they had to have a command and a four digit password in the body of the message. Then the system sent back a response to the phone number that came in on their message, in the caller id header.
If the command wasn't valid, or the password wasn't valid then it sent back a message to that effect.
If you'd like to see it I can post the code, but it will take me a day or so to find it, it's on a back up disk somewhere.

Jerry.