Back in November 2001, a Washington Post article by Yuki Noguchi described how wireless phone providers test the cellular network for quality and outages. It's an excellent article:

The carriers contract with companies such as the one Rutledge works for -- LCC International Inc. in McLean -- to find the weakest spots in their networks and to compare their service to the competition's. Most also conduct their own tests.

Other than customer complaints, the drive tests are the only way Verizon Wireless can check its network, said John Johnson, a spokesman for the country's largest wireless-phone firm, which typically tests 2,000 miles a month in the Washington-Baltimore area. If a call is dropped, or if it fades out, Verizon can reprogram software in the network to make the signal stronger or plan to build another cell tower to increase its coverage, Johnson said...

Rutledge's van has six phones -- one for each of the Washington area's major wireless carriers: Verizon, Cingular Wireless, AT&T Wireless, Sprint PCS, Nextel Communications Inc. and VoiceStream Wireless Corp. Every two minutes, each phone is directed by a computer program to make another call. A database keeps track of the strength of the signal, the quality of the transmission and whether the call runs into trouble...

Rutledge has worked for LCC for six years, and he said he's mapped nearly every city in the country that way, except the ones in New Mexico, which he figures is simply a matter of time.