They call saying they're from the Internal Revenue Service, and under threat of arrest they demand payment. But they're scammers, and there are more of them than ever before — a lot more.

Complaints about people impersonating IRS agents have surged over the past year. During the first six months of 2014, the Federal Trade Commission received more than 12,000 complaints about such scams. That's 36 times the volume logged during the first six months of 2013, according to a BuzzFeed analysis of data obtained from the FTC via a Freedom of Information Act request.

Monthly totals keep reaching all-time highs. In June the FTC received 3,742 complaints — more than they received during all of 2013.

Even so, "complaints are probably just the tip of the iceberg because many people don't know to even call the FTC," said Steve Toporoff, an attorney for the FTC's Division of Privacy and Identity Protection.

If the Internal Revenue Service finds a problem with your taxes, they'll send you a letter. But to those unaware, tax-collection scam calls can feel incredibly threatening. The scammers pose as either IRS agents or collectors. Heather Gray and Frank Jefferson are among the many pseudonyms used. They sometimes recite personal information, such as the last four digits of a victim's Social Security number, and often claim to be able to send federal agents to victims' homes, or to deport them.

These two scam messages were left on a BuzzFeed reporter's personal voicemail:

If the target does pick up the phone, the caller typically demands that they send a prepaid debit card, wire transfer or a credit card number, and threatens non-compliant targets with arrest, imprisonment, or deportation. Some victims have reportedly lost thousands of dollars to the scams.

To be sure, some portion of the sharp increase in complaints can likely be attributed to IRS and FTC consumer-outreach efforts. The IRS issued what appears to be its first warning about this type of scam in October 2013 and reiterated it during tax season in April. The FTC also published a blog post in April on the subject. Toporoff said that a typical FTC blog post might receive two or three comments, that one received nearly 1,000. More complaints can spur more action, which informs more consumers, which in turn spurs more complaints.

But there's reason to believe that these scams are actually becoming more prevalent. Even before the IRS's October 2013 warning, consumers had started filing more complaints than any time in the previous two years. The Treasury Inspector General for Tax Administration has called the recent spate "the largest scam of its kind that we have ever seen." As of March, the agency said it had received reports of more than 20,000 contacts costing consumers more than $1,000,000. (While the FTC receives complaints about these issues, TIGTA is tasked with the prevention and detection of fraud within the IRS.)

It's unclear whether federal agencies are cracking down on the scammers. When asked about enforcement the FBI and TIGTA declined to comment. A spokesperson for the IRS said that the FTC was the leading agency for telephone scams.

Though some IRS-impersonation scammers contact their victims via email, phone call–based scams have accounted for the vast majority of complaints. Callers typically use voice-over-IP (VoIP) services, claiming numbers associated with authoritative-looking area codes, such as D.C.'s 202 (the most common) or San Francisco's 415. But there's no way of knowing where the calls are actually originating.

One VoIP service popular with scammers, magicJack, says it's been working to shut down suspicious accounts. "If magicJack is made aware that a consumer was using a magicJack device inappropriately we will immediately shut them down," Mary Kennon, the company's vice president of customer experience, told BuzzFeed.

BuzzFeed sent magicJack a list of 11 phone numbers that appeared to have used their service. Ten had been suspended earlier this year, according to Kennon. The remaining phone number had not yet been suspended but quickly was.

The recent wave of IRS scams has prompted complaints from consumers in every single state, as well as D.C. and Puerto Rico. Overall, Californians have sent the most complaints. Per capita, the top states are Delaware (9.0 complaints per 100,000 residents), Nevada (7.1), Hawaii (6.9), Massachusetts (6.3), and New Jersey (6.2).

What to do if you think you're being scammed: Experts recommend that you hang up the phone and report the call to the FTC. If you're worried about your tax return, hang up and call the IRS directly at 1-800-829-1040.

Have you been harassed by an IRS impersonator? Let us know in the comments below or email John Templon at [email protected].

Note: The data and analysis used for this post are available here.