AT command for hang up call automatically at hardware level

I'm using sim900b to control home appliances via sms . But sometime incoming call interrupts the communication , I used "ATH" command each time I'm getting a new incoming call . When sim900b gets a incoming call , it sends incoming call status to microcontroller(atmega 328p) , and after checking this microcontroller sends "ATH \n\r" to sim900b and then it disconnects the call .

I want to is there any alternative way available so that sim900b will automatically disconnect the call by itself , so that I wont need to send any command from microcontroller to disconnect the call ?

Are you trying to control home appliance by just dialling SIM900B ? (never heard about SIM900B) If yes, how do you detect the number of the caller before controlling devices ? If you are not checking the number of caller then can call from any mobile to the system controls the device ?

Mention Compiler used and post code.

Oops. I misread your post. You are controlling devices using SMS but incoming call to SIM900B is affecting it. Why call comes to the device. If you are using a new SIM which is only used for SMS controlling device and have not provided the number to anyone then why does it and from where does it getting the calls ?

Is it getting calls from Service Provider ? I have not experienced such a problem. How are you checking that new SMS ? using new SMS notification ? When are you clearing the uartBuffer ? What is the size of UART buffer ?

Got any h/w handshake enabled? like DSR/DTR CD RTS/CTS?
That's the only instant way.