You probably wouldn't be able to do it without opening up the mobile and hooking up a circuit to the existing mobile keypad. I had to do this with my "car alarm dialer" i designed that rings my other mobile phone automatically if the car alarm tripped. I had to design a circuit to control some reed relays to simulate the key presses (the circuit controlled the mobile as if I was there pressing the buttons). The circuit would activate automatically as soon as power was applied to it via the car alarm's siren. Just thought I'd express my thoughts. Others might have better ideas of going about this.
Jason.