Nortel Networks and Cisco Systems both defended its own approach for voice-over-IP at VoiceCon2004 in Florida.
Nortel is a traditional voice switch supplier and has products which adapt these traditional networks to IP.
Cisco, meanwhile, sells pure IP products to handle voice, treating it as just another data stream.
Michael Frendo, vice-president of voice systems engineering at Cisco likened the Nortel-style technologies to driving a car with a horse tied to the rear - just in case the car fails.
Philip Edholm, chief technology for enterprise solutions at Nortel, acknowledged that a company can go with a pure IP system, but he questioned whether it should. He said a long list of Cisco customers have installed Cisco IP voice capability only to back off, including Merrill Lynch.
Frendo countered that only a handful of customers had done so - not because they did not like Cisco's products, but because they wanted a different architecture.
Merrill Lynch senior official said last year that the firm had decided not to use the Cisco system because it did not want to lose voice capabilities if its data network crashed.
Edholm also contended that Cisco's approach to VoIP seems to omit the idea of a "softphone", which relies on the computer to make calls and does away with the need for a handset. With such an approach, a user can click on an icon on a screen to place a call, without needing a phone number or even a person's name.
Cisco sells several telephone handsets with small screens which, Edholm claimed, was too small for weak eyes to read.
Edholm said Microsoft and Intel have signalled an interest in using session initiation protocol in softphones as a means of starting voice sessions with PCs.
Frendo said, "The softphone is in our future," adding that without pure IP at the endpoint, it is not possible to run video over phones.
Cisco has a large percentage of the US market for voice systems of all types, considering that it is a relative newcomer, said Allan Sulkin, an analyst at TEQConsult Group.
"Cisco has pushed the other vendors to step up their programs," he said. "The market has been shaken and stirred by Cisco, and Avaya and Nortel have to watch out."
Matt Hamblen writes for Computerworld