Incidents of hackers convincing cellphone companies to hand over control of user numbers have increased 150 percent in three years.

As cryptocurrency trader and investor Chris Burniske told the New York Times this week, the speed with which attackers compromise user devices with assumed links to virtual currency holdings is cause for great concern.

“My iPad restarted, my phone restarted and my computer restarted, and that’s when I got the cold sweat and was like, ‘O.K., this is really serious,’”  he told the publication about the episode in 2016 which saw hackers drain $150,000 worth of coins.

2,658 occurrences of theft via phone number came to light in January 2016, up from 1,038 the same month three years prior.

Earlier this summer, users of Classic Ether Wallet suffered a similar fate after an attacker convinced its German ISP 1&1 to hand over control of the site’s domain.

“It’s really highlighting the insecurity of using any kind of telephone-based security,” ShapeShift chief information security officer Michael Perklin added following multiple reports of its own users being affected.

Cointelegraph reminds readers never to store cryptocurrency holdings in easily accessible format. Coins held in hot wallets should be kept to an absolute minimum, with the majority in cold storage such as a hardware wallet.