Q. How does telephone spoofing work, and is it illegal?

A. Caller ID spoofing services let callers hide their real phone number, and instead display another number on the recipient’s caller ID service, along with the name of the company or individual registered for that number. Such services, widely available on the Internet and costing as little as $10 per hour of calling time, are legal and have traditionally been used by collection agencies, police, private investigators and others who want to hide their identities.

But increasingly, scammers are using spoofing services to commit identity theft. They pretend to be calling from a bank, credit card company or a government agency, and the recipients may feel safe revealing personal information such as bank account and Social Security numbers because the name and phone number of the legitimate company or agency appears on their caller ID.

Once a scammer has subscribed to a spoofing service, getting the victim’s caller ID to display a fake number is easy. The spoofer simply enters the recipient’s number and the fake caller ID number on a Web form or telephone touchpad, and the service completes the call.

The practice has become even more widespread with the growth of the technology known as voice over Internet protocol. VoIP phone services such as Skype and Vonage use computer addresses instead of actual phone numbers to connect calls over the Internet. That means their subscribers can choose any area code and phone number that’s available in the entire country, and scammers can take advantage of that flexibility to choose a deceptive number.

Florida is the only state that has banned caller ID spoofing, making it a first-degree misdemeanor, and some other states are considering similar laws.

No matter where you live, people who have signed up for the National Do Not Call Registry can file complaints about unsolicited spoofing calls with the Federal Trade Commission or their state attorney general’s office.

The bottom line: Because spoofing is easily done and enforcement has been lax, you should never provide sensitive information to any caller. Instead, look up the number of the company that claims to be calling and dial it yourself.

Sid Kirchheimer is the author of Scam-Proof Your Life (AARP Books/Sterling).