The T-Mobile (NASDAQ:TMUS) network suffered a long outage on Monday. Millions T-Mobile and Sprint subscribers across the country were unable to make or receive voice calls and SMS texts for a 12-hour period, starting near noon EDT.

Federal Communications Commission (FCC) chairman Ajit Pai called the outage "unacceptable" and vowed to launch an official investigation of the issue.

A man rests his forehead on the floor, holding fistfuls of tangled network cables.

Image source: Getty Images.

The inside story

According to T-Mobile CEO Mike Sievert, the voice and text network outage was caused by a botched network maintenance procedure, a part of the three-year project to integrate Sprint's network infrastructure with T-Mobile's. Company representatives suggested using alternative calling and messaging methods such as WhatsApp or Signal during the outage since the data network was fully operational.

"This is an IP traffic related issue that has created significant capacity issues in the network core throughout the day," Sievert said in a prepared statement.

Security expert analysis

Early complaints suggested that T-Mobile's outage might have been caused by a distributed denial of service (DDoS) attack, but network security specialist Cloudflare (NYSE:NET) CEO Matthew Prince debunked that theory long before T-Mobile's official explanation came along.

The DDoS graphs that inspired the attack hypothesis are marketing tools used by companies like Cloudflare in order to sell DDoS mitigation services, Prince explained in a series of tweets. They are designed to always look frightening, reflecting the harsh reality that DDoS attacks are a common issue.

"Our team know the network operators at nearly all the other major Internet services and platforms and none of them are reporting anything anomalous," Prince said.

Except for T-Mobile, of course. Prince concluded that the network's network issues were "almost certainly entirely of their own team's making."

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