when i do this proj in past,,, i use differnt algo to u ,,,,, eg init UART once, setup sim900 once, then loop waiting 4 chars to start, then loop reading chars into buffer or state machne testing 4 cmd start char eg #, then action on recd cmd (led, relay,,,), then back to loop waiting 4 chars. u shld not need delay_ms after UART1_Read!