jilly,
they don't need to have the telephone in hand - all the information they need (calls made, received, tower check in reports) can be obtained under subpeona from the phone service provider. The phone itself is not necessary.
I posted this link earlier - it explains how the phone system works...
http://www.mindfully.org/Technology/2005/Cell-Site-Location-Smith14oct05.htm
When a cell phone is powered up, it acts as a scanning radio, searching through a list of control channels for the strongest signal. The cell phone re-scans every seven seconds or when the signal strength weakens, regardless of whether a call is placed. The cell phone searches for a five-digit number known as the System Identification Code assigned to service providers. After selecting a channel, the cell phone identifies itself by sending its programmed codes which identify the phone, the phones owner, and the service provider. These codes include an Electronic Serial Number (a unique 32-bit number programmed into the phone by the manufacturer), and a Mobile Identification Number, a 10-digit number derived from the phones number.
The cell site relays these codes to the mobile telecommunications switching office in a process known as registration. The registration process is explained in the Department of Justices Electronic Surveillance Manual:
Cellular telephones that are powered on will automatically register or re-register with a cellular tower as the phone travels within the providers service area. The registration process is the technical means by which the network identifies the subscriber, validates the account and determines where to route call traffic. This exchange occurs on a dedicated control channel that is clearly separate from that used for call content (i.e. audio)which occurs on a separate dedicated channel.