Your phone number's area code gives other people a clue to where you live, or have lived in the past. Startup company Loc-Aid can use your full phone number to figure out exactly where you are right now.

The service has caught the eye of banks and card issuers interested in checking where their customers are—as a way to reduce fraud—and of retailers interested in sending deals to people nearby.

"We can locate any one of the more than 350 million devices on the major U.S. and Canadian carriers in real time," says Rip Gerber, founder and CEO of Loc-Aid, based in San Francisco.

You can test Loc-Aid's ability to find your phone using this Web demo. It takes from five to 20 seconds to get a location fix for the device associated with a phone number. "The companies using our service already know their customer's phone number and just need to get permission from them to use that to find their location—that's usually done by SMS," says Gerber.

A person might be prompted to allow such tracking when they use their bank's mobile website or app, says Gerber. He says that Loc-Aid will ensure that a request for permission is never buried in the terms and conditions of a service and that a user is always told exactly how his or her data will be used.

Loc-Aid's service is possible because it developed technology and made agreements that aggregate the phone-tracking abilities of every major U.S. and Canadian cellular network, which are legally required to have technology that spits out the location of a device making a 911 call.

Devices are located using the signal strength of nearby cell towers. The accuracy varies from hundreds of meters in areas with few cell phone towers, such as rural areas, but can be to within a block or even an individual building in urban areas.

Earlier this year, Loc-Aid added the final major wireless carrier—T-Mobile—needed to provide universal coverage for the major services in U.S. and Canada, covering roughly 95 percent of North American cellular devices.

"Now we have enough of a footprint that the service becomes very interesting and useful," says Gerber. Loc-Aid is running pilot projects with one major credit-card issuer and several banks; these companies are interested in locating users in order to cut fraud. It costs such companies hundreds of millions of dollars to run a call center to contact people when their cards trip fraud-detection alarms. Checking the location of a person's device may offer a quicker and cheaper way to identify potentially suspect transactions.