Around 900,000 routers across Germany were hit by the outage which started on Sunday, a company spokesman told DPA on Monday.
The routers connect customers not only to the internet, but also to telephone and television services. The spokesman explained that the problem was not with the network itself, but rather with identifying routers upon dial-up.
The company is now looking into evidence found by IT analysts that the connection problem may have been due to an outside attack rather than a normal system failure, Telekom said late Monday morning.
"We have found the first indications that we were possibly victims of a hacker attack," a spokesman said.