I can use any sim card to send and receive sms messages. I already developed a vb program to do that, the only problem now is how to interface it directly to 8051.

here is the link:
http://www.vinculum.com/prd_vdip1.html

I really like this vdip over the rs232 gsm modems because it is very versatile, i can also add other peripheral devices like a webcam, a flashdisk, a printer, etc.