You can't use a mobile phone, the signal isn't good enough once you hide the phone under the dash or in the boot etc
E-bay is your friend, something like this
http://www.ebay.com.au/itm/OZ-TK104-AU-GPS-GSM-Tracker-System-Car-Vehicle-Spy-Hidden-Device-60-Days-Standby-/321333664946?pt=AU_Electronics_GPS&hash=item4ad0fa9cb2
They have inbuilt aerial's for GPS, GPRS & GSM (GPRS being mobile phone reception or GSM can't remember which) and come with external extensions for both for better reception.
Sim cards are another problem as most GPS trackers use the old G2 system and you will need a backwards compatible sim
I have an ALDI simcard. They cost 5$ yes you heard me, only FIVE DOLLARS!!! and last a full 365 days (no monthly cost or charges, and you can recharge online to extend the time)
The system works on sending sms messages to a mobile phone you chose with updates of position etc, SO..... every update cost you a sms charge. (ALDI's sim only cost 12c per sms)
Now the down side,
1 Only works out doors, (If the car is parked in a shed say, it might give an update and if it does it will be wildly inaccurate. this does depend on location)
2 if you lose either GPS or Mobile signal it no longer works (underground car park, or out in the country where mobile reception is patchey)
3 Theifs carry jammers to block the mobile signal (while illegal, so is stealing your car)