You can achieve it by using a DTMF decoder IC. But you will need two cellphones for this design. One attached to your robot(it will be the receiver) and the other will be used to control your robot(transmitter).
The basic idea is that you will make a call to the receiver and press a button. This will produce a corresponding tone at your receiver. The DTMF, attached to your receiver, will decode this tone and convert it into its equivalent binary digit. You can use this to control your robot.