Internet voice market leader Vonage Holdings is asking a Delaware court to invalidate patent infringement counterclaims being filed by telecommunications equipment maker Nortel Networks.

The move is the latest chapter in a high stakes battle being fought around VoIP, a technology that is revolutionizing the telecom market, but has lately involved more input from lawyers than innovators.

Nortel filed the lawsuit as part of its amended response to a three-year-old legal action taken by Digital Packet Licensing (DPL) alleging patent infringement against Nortel.

Vonage inherited DPL’s legal action against Nortel when it acquired DPL in June 2006.

The DPL suit, which was filed in Texas, alleges infringement by Nortel of three patents that govern the compression of packetized digital signals commonly used in VoIP technology.

Nortel recently asked the Texas court to amend its response to include counterclaims of patent infringement against Vonage involving three Nortel patents.

Vonage said that the Nortel patents, which govern aspects of phone data management, should not be part of Nortel’s response since they are unconnected to the original case.

“At this time, it is unclear if the District Court in Texas will allow Nortel to amend their original response,” said Vonage spokesman Charles Sahner. “Vonage opposes any attempt by Nortel to amend its response in the Texas case as untimely and unrelated to the current action.”

Vonage has been the target of a number of lawsuits involving the movement and management of IP based phone calls across the public network.

Vonage suffered a massive blow in March when a federal jury ordered it to pay Verizon Communications $58 million in damages for infringing on the telecommunications giant’s patents.

Vonage was also ordered to pay a 5.5 percent royalty to Verizon on future Vonage sales.

Vonage, which said it faces bankruptcy as a result of the case, is appealing the decision while seeking technical workarounds to get out of Verizon’s legal target zone.