The lawsuit comes just days after Vonage settled a patent-infringement lawsuit with telecom provider Sprint Nextel.
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 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 $80 million. As part of that agreement, Vonage agreed to license VoIP patents from Sprint, including more than 100 patents covering technology for connecting calls from a traditional phone network to an IP network.
Vonage is also in the process of resolving a patent infringement dispute with Verizon. Earlier this year, a court found Vonage infringed on Verizon patents and ordered an injunction that could have prevented Vonage from signing up new customers. Vonage won an injunction staying the order and is appealing the original infringement ruling. Vonage in August said it was close to rolling out workarounds for two of the three patents Verizon claimed.
Vonage is one of the largest independent VoIP providers in the U.S. with nearly 2.5 million customers.