AT&T has sued VoIP provider Vonage, seeking damages for alleged patent infringement, just days after Vonage settled a similar suit with Sprint Nextel.

In a filing with a Wisconsin court, AT&T alleged that Vonage wilfully infringed an AT&T patent related to telephone systems that allow people to make VoIP calls using standard telephones.

In the 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 8 October that it settled with Sprint Nextel for $80 million. 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.