To clarify a point. You don't need to be on a call all the time.
When a phone is turned on, it looks for a network cell. It then send the ID of the SIM card with the ID of the phone, which in turn updates the network/phone database.
So when you call a number, it looks for that phones cell, then sends a "ring ring" to the phone, which is linked to the sim card.
What is also sent to the phone when it is turned on is what other cells are around it.
The phone keeps monitoring those other cell stations. When you move around and it gets a better signal from another cell, it would then use the better strength cell staiton as it's "base"
This is an ongoing process and that's what those stupid sounds that you get on your radio station just before the phone rings or it changes base stations (Geeks: this is basically your phone doing a ping/network scan)
SOOOOO what does this mean? At any given point in time there are about 3/4 stations that know where and at what strength your phones is.
Simple trig would allow you to at least pin point the phone to "about" 25 meters.
Do this at say 4am on a tuesday and you could probably find your car.
If phone companies would do this another whole kettle of fish.
Oh, which brings up another point. The telco providers have had the technology (cuz it is built into the GSM networks) to trace and block IMEI numbers of mobile phones.
It isn't worth it too them though as one person signs up for 12 months on a contract, loses it, either pays out contrct or gets new phone, meanwhile stolen phone is also contected. So they get two people making calls on the one contract. Schemeing buggers.