hi shresthi ,
please post the heart beat hardware and software coding ,if have problem in sim 300 module pleaes check sim300 using RS232 follow the bellow test for check a sim 300 module
1.connect the RS232 to computer
2.open hyberterminal
3.set com porty
4.set the baudrate 9600
5.flo control bit none
6.harware control none
7.chack these command AT
8.sim300 replay OK
THAT MEANS SIM 300 IS WORKING