You have to solve a couple of problems here:
1. How does this pager system get its information and can you somehow get that information out of the pager system?
2. How does, or is there, a GPS program that allows you to auto-program a route into it? For example, can you put a location into your favorites file that is the current response location so that you can simply hit the 'route-to' button? Can you go further and interact with the program to tell it to build a route for you?
3. Standby of the system. Will you have the system on standby or will you have it hibernate or cold boot? Time is the main variable here but also if you are constantly waking up your pc to have it up and running when you get in the car, what happens if you miss a call or the pc wakes up for some unknown reason? It could drain your battery leaving you with a dead batt just when you need to respond to a call.
I'd investigate number 1 and 2 first. You can probably solve number 3 later on but first you must get and process the information for the call.