A T-Mobile customer is suing the carrier over a cryptocurrency heist.

Carlos Tapang claims that hackers broke into his cryptocurrency account because T-Mobile was fooled into transferring his account to AT&T. The hackers then used Tapang's wireless account to access his cryptocurrency account and sell virtual currencies for 2.875 in bitcoin.

Bitcoin was worth around $8,000 around the time of the account transfer, but reached a high of $19,000 the following month.

The lawsuit, filed on Sunday and first spotted by Law360, seeks damages.

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The incident is a classic case of identity theft. Hackers who obtain your personal information—like birth date, Social Security number, and address—can go to a company and sometimes successfully impersonate you. In this case, the hackers targeted Tapang's wireless account, which was registered with one of his cryptocurrency accounts.

Tapang, a resident of King County, Washington, noticed the theft when his phone lost its connection to T-Mobile on Nov. 7. He called the carrier, and learned that T-Mobile had canceled the service and transferred his phone number to AT&T.

"More specifically, unbeknownst to Mr. Tapang, T-Mobile had transferred control of his phone number to a device under the control of someone else," the lawsuit claims.

It took T-Mobile a day or two to retrieve the phone number from AT&T, the lawsuit says. But by then it was too late. The hackers had changed the passwords on one of Tapang's cryptocurrency accounts, and drained the funds inside.

The lawsuit accuses T-Mobile of failing to stop the identity theft, even as Tapang had placed a security measure on his phone number that should have thwarted the hackers. Prior to the heist, Tapang had enabled a PIN number that was supposedly needed to transfer his phone number to another carrier. But despite this, the hackers still tricked T-Mobile agents into porting Tapang's phone number to AT&T.

As a result, the lawsuit accuses T-Mobile of failing to train its employees to prevent the identity theft schemes. It also notes that other T-Mobile customers have complained online about similar scams.

So far, the carrier hasn't commented on the lawsuit. But in recent weeks, T-Mobile has reportedly been warning customers about identity theft schemes that involve transferring a phone number to another carrier. To ward off the threat, the company has been encouraging customers to create a PIN/passcode with their T-Mobile accounts — the same safeguard Tapang used to protect his phone number. Nevertheless, enabling the PIN is still a good idea.

This article originally appeared on PCMag.com.