I was thinking about doing the exact same thing with an old but indestructible Nokia 5070 phone.
I have already ordered a $1.25 fbus cable. I think that a serial converter needs to be used
(max-232) and then part of the problem is that programming trunks is not that easy...
but where is the fun if you take that out? I want to make a simple alarm watching a door
with a hall effect magnetic sensor and also use a classical IR sensor. Also, I plan to user the
nokia battery to power the whole thing, and the original nokia charger. This alarm will be totally
silent. If there is a break in, only an SMS will be sent to my phone and maybe to my mother's.
Also I was thinking that it should be possible to make arduino answer an incoming phone call,
if you want to listen to whatever is going on at your place.
As I live in Argentina, most GSM shields will not work here (must be 4 bands or at least
have the 850 Mhz band)... the ones available are way too expensive. This experiment will cost
me close to nothing. The phone has been lying around for years... the rest of the parts I already
have.
The beauty of this is that you can do a lot of things. I want a custom alarm, just because I cannot
think of another use, but if you live in the country side and you want to control remotely things
that are really far away, like water pumps or some other stuff, this can be an option.
There is a lot of info about Nokia trunk programming in the web. I can post advances when my
cable arrives and I start playing with this.