Get yourself a pay as as you go SIM with the TK103, setup an online account with the telecom service provider so that you can top up whenever needed and you'll be good to go.
Don't forget to get the tracker to respond to your mobile with a message, to keep the SIM card 'active' as your not making phone calls. As the service provider normally will terminate after 3 months of dormant use!
Lots of useful info online and I think the TK103 has been upgraded with another model since I bought one, not sure now.
Clever these tracker devices, they also have a movement sensor so if anybody tries to lift the car on a flatbed or even move the pride and joy, you will be notified immediately!