Suspicious behavior on T-Mobile US Inc.’s network devices tipped off the company to a breach that was potentially part of a sprawling cyber-espionage campaign that has raised urgent questions about the exposure of a critical sector of the economy.

T-Mobile says the Chinese "Salt Typhoon" hackers who recently compromised its systems as part of a series of telecom breaches first hacked into some of its routers to explore ways to navigate laterally through the network.

T-Mobile has severed its connection to the unnamed wireline provider, which it suspects may still be compromised.