Seen this a few years ago… why havent we all done this (shamelessly cut n pasted from a random forum) ?
Mobile phone bike alarm DIY How To By popular request 
Alarm Phone - a set on Flickr
Basically all you need to do is replace the green key on a standard (non-touch screen) phone with a “trembler” switch.
The switch you need can be found here Assemtech | Process Control | Sensors and Transducers | Nonmercury Tilt Switches | Non-Mercury Tilt, Vibration & Movement |S1425
Just connect this across the tracks on the PCB. On standard mobiles, if you press the green key twice it dials the last number dialled.
So what you do is put the number of your mobile in and call it (by shaking the phone). In your mobile you create a contact called “Bike Alarm” and enter the number of the alarm phone.
I recommend getting a new PAYG SIM for the alarm phone.
To use the alarm, turn it on and hide it on the bike. If anybody messes with it, the alarm will phone you up, so you get a phone call from “Bike Alarm”. At that point you can either grab your Machete and run, or answer the call and have a listen, or do whatever you feel.
It’s best to answer the call and hang up immediately, because you have to pay for whatever the alarm phone does, so if it calls your mobile and you don’t answer it’ll go to voicemail for 2 or 3 minutes before your phone hangs up.
If you want to test if the alarm phone is in network and working you can call it up. If you here the ringing tone your set. Of course you need to set the ring tone on the alarm phone to silent.
