I guess you could do all that gsm, but all you really need is to trip a relay or some sort of switch to enable the heater. Do you need to turn it off with the cell phone?
I remember there was a thread on here where someone used the power to the speaker to drive a transistor that drove a relay. Just make sure your ringtone is a constant ring and it should drive a transistor, which you could add a latching circuit to if you didn't need turn it off with the phone.
I wish I could find it, but this search feature on this forum doesn't work worth a crap for me.