Hi
that is a good idea, I've done something similar a few times. I'm not to sure how your phone works so cannot give any real advice. But I can give you some general information.
GPRS is a commonly mis-understood. Some idiots believe that it is a location technology :lol: . In actual fact, as you probably know, it is the General Packet Radio Service, it is GSM but used in a slightly different way. It is an IP based system, when using GPRS modules you form an IP header around your packet of data and then present that packet to the module and it takes care of the rest. The data is then sent over IP so if you have a broadband connection there will be no cost in recieving the data. The only cost will be on your mobile phone bill. The network providers are using the internet to route traffic because it is cheaper and less costly to maintain. Soon, all new phones will make audio phone calls using GPRS and the internet. VoIP is the technology of the times and a lot of money is going into it development at the moment.
GPRS is cheaper than standard GSM, charges vary but 50p/MB is about the going rate. One MB of tracking data can last for a year if you wanted it to. I assume that you are using a data enabled sim and calling to through a modem. A novel idea, would be to do as you are at the moment, but instead of spending 5p/min and have a real time link get it to call you, or you call it for a short burst every so often, this will cut the call costs.
Using two phones is not really a great idea, it will work but it is not a neat solution.
GPRS offers a neater solution as it is "allways on", and so there are no call up time, Once set up, you could program your PIC to send a small packet of location data every 10sec, 30sec 1min etc to provide realtime tracking. The IP header would contain the destination, a port on the IP address of your router.
hope this helps a little.