Data collected by Brix Networks' TestYourVoIP.com free voice quality testing site found that one in five VoIP calls were of an "unacceptable quality".
The TestYourVoIP service has been used to gauge the quality of over a million calls from users around the world since its launch in March 2004.
The data shows a consistent decrease in voice calls that achieved 'acceptable' call quality or better over the past 18 months.
"Over the past few years, the global market for consumer VoIP services has grown to nearly 20 million subscribers. These results from TestYourVoIP.com indicate that during this same period internet call quality has declined," said Kaynam Hedayat, engineering vice president and chief technology officer at Brix Networks.
Hedayat stressed that internet phone providers need to concentrate on the causes of poor call quality, such as lost data packets.
The TestYourVoIP service can be conducted via a web browser with Java installed or through a gadget plug-in for Google's Desktop software.
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