What SMS can send to switch LED?
The code appears to be looking for an SMS that says #a0 or #a1. The logic doesn't look very robust, though. It reads a character, then waits 10ms and tries to read another. What does SIM900.read() do if there is no next character? Will it return something? Or wait until there is another? If it waits, the code can get out of sync with incoming, badly-formed commands.