AT&T is the latest telecommunications company to sue Vonage Holdings for patent infringement. Earlier this month, Vonage agreed to pay $80 million to Sprint Nextel to settle that patent litigation.

AT&T and Vonage said they have been in discussions for months in attempts to settle their patent differences. "It's our preference to settle disputes through negotiation rather than litigation," said Sharon O'Leary, Vonage's chief legal officer, in a statement. "We will continue to work toward an amicable solution."

Verizon began the series of lawsuits against Vonage and won a jury verdict in March. Since then, however, Vonage has fought back, claiming it has developed workarounds for two of the three patents it was found by the jury to have violated.

Earlier this month, Vonage asked the U.S. Court of Appeals for the Federal Circuit to review earlier rulings against the VoIP firm. "This move represents the next logical step for Vonage in managing this litigation and continuing to move our business forward," said O'Leary. "We recently settled our case with Sprint, and continue to explore all legal options available to put the Verizon litigation to rest."

Vonage has said it settled another patent case with Klausner Technologies, which had been seeking $200 million.

Taken together, the patent litigation against Vonage covers a wide range of properties from voice mail to interconnect links and includes compression and packet technologies. A major question is whether the established telecommunications firms will be able to or will have the desire to institute patent infringement suits against other VoIP players.

AT&T filed its suit against Vonage in a Wisconsin federal court last week. According to media reports, the single patent in the case involved the use of a standard phone linked to the Internet.

Vonage hasn't fared well with juries, losing both the Verizon and Sprint cases in jury trials.