Voice over internet protocol (VoIP) is continuing to improve in call quality and reliability, according to a new study.

Although the technology is still in its infancy, VoIP is rapidly closing the gap on traditional public telephony networks, research by Keynote Systems found.

"Our key finding here is that there has been significant improvement for VoIP services, but there is still a difference in terms of quality of calls," said Rajeev Kutty, VoIP product manager at Keynote.

The study by the technology testing firm was conducted by making more than 125,000 calls between New York and San Francisco over a month.

Although some delay and call completion problems were recorded, the best-performing VoIP firms had a completion rate of over 99.5 per cent.

This suggests that potential VoIP customers should investigate the performance of providers before settling on one which best meets their needs.