The Associated Press report this makes AT&T the third major phone company to sue Vonage, which until recently was a leader in selling phone service that rides the customer's broadband connection.
In a filing with the Securities and Exchange Commission, Vonage said AT&T filed the lawsuit in US District Court in Madison, Wisconsin, the report said.
It said it has been in discussions with AT&T to resolve the dispute but can't guarantee the case won't go to trial, the Associated Press report further said.
The single patent in question, filed in 1996, appears to broadly describe the idea of routing telephone calls over data networks like the Internet.
The listed inventor is Alexander Fraser, AT&T's former chief scientist.
Two weeks ago, Holmdel, N.J.-based Vonage settled a patent dispute with Sprint Nextel for $80 million.
A suit filed by Verizon Communications is still in the courts, but Vonage suffered a significant setback in that process in March, when a jury awarded Verizon $58 million in damages, plus future royalties, after finding that Vonage violated three Verizon patents.