you should know about actuators like electric motors and their driving circuits, microcontrollers programming and interfacing, GSM AT commands, robot mechanical design, i recommend for you this book "Robot builder bonanza".
good luck
You may connect the earphone output of the mobile used to receive, to a DTMF decoder like MT8870, which will decode the dtmf tones sent by the caller and then switch the functions of the robot respectively. Cheers