cellphone automatic dialling

hi,
i'd like to make a cellphone to automatically dial a preset phone number, without any key pressing. it would be something like this:
the phone would be connected to the car door so when it was opened it would somehow send an input to the cellphone which would then dial a number.
can i connect anything to the cellphone or replace the keypad with something so that, for example, it would receive an input from the interior light (when the door was opened) and the "device" would emulate pressing the keys?

or, could i mess with the cellphone software so when the car was turned on it would turn on the cellphone and then automatically dial a number? (not a question related to electronics, but way easier and maybe some of you know how)

i don't know what i'm talking about, if it's possible or how hard can it be... i'm more than noob

any ideas?

Pimpi, I'm pretty impressed about your idea, actually some of may colleagues built a device that works exactly like that. They call it TXTalert GSM alarm System and it's already out in the market.