More than 27 percent of VOIP subscribers who are likely to change providers cite network quality as their primary reason for a switch, according to Agarwal. Improved customer service and better plan prices are also critical to why subscribers would switch service providers, he said. In addition, more than 12 percent of all VOIP subscribers are likely to leave their current VOIP service provider for another supplier within a year, said Agarwal.

"The battle is not over," he said. "Just because Vonage got these subscribers, doesn't mean they're going to stay with Vonage."