c programming, sending sms

[JAMES SCRYMGEOUR]

hi
i have been writing a program to receive a message read the message then run a 
program, before replying to the sender with the response.
I have managed to set up the program to read and respond, but am having 
problems sending an sms message back, setting the users number as a variable 
and the numbers as a variable,
i am programming in C, any advise would be greatly appreciated
many thanks
James