America’s leading telephone operator, AT&T Inc., filed an intellectual property lawsuit against the troubled VoIP provider, Vonage Holdings Corp yesterday, seeking damages for alleged patent infringement.

AT&T’s lawsuit, filed at a U.S. District Court in Wisconsin, alleges that Vonage willfully violated an AT&T patent concerning telephone systems that allow customers to make VoIP calls using standard telephone equipment.

AT&T claims that it tried to reach a licensing agreement with Vonage, but was unsuccessful, forcing it to file suit against its smaller competitor.

This latest legal woe comes just days after Vonage settled a similar dispute with wireless carrier, Sprint Nextel, for $80 million. Vonage is also fighting a losing battle against New York-based telephone giant, Verizon Communications, to which it lost another massive patent infringement suit earlier this year.