Dtmf based robot control implementation is very easy . Connect a audio jack of any cell phone to you microcontroller(mc). Mc monitors it. After you are in call wit tat phone . On the cal. Different buttons from caller ex 1 2 3 would result in different tones at recieves phone . Which can monitored by a microcontroller . Assign different tasks for different tasks for different frequencies for your robot . Simple =) proper implementation would get you many controls . You can get the most out of it . Because gsm mobie range coverage cant be compared to any rc.!