On the heels of Sprint’s success against Vonage, AT&T follows suit:

In a filing with the U.S. District Court for the Western District of Wisconsin, AT&T alleged that Vonage wilfully infringed an AT&T patent related to telephone systems that allow people to make VOIP (voice-over-Internet Protocol) calls using standard telephone devices.

In the legal filing, AT&T said it tried to reach an agreement with Vonage to license the patent, but failed, which forced the lawsuit.

Vonage announced on Oct. 8 that it settled its suit with Sprint Nextel for US$80 million.