Hi,
Learn about GSM interfacing from engineers garage itself.
/embedded/pic-microcontroller-projects/interface-gsm-module-call-message-circuit
Now all you’ve to do is yo wait till a pin goes high before executing the message sending code.
void main ()
{
while (1)
{
while (PIN==0); //program will loop in this line itself only until the pin become logic high
/********************
Your message sending code
********************/
}
}
I hope this helps.