What was a significant barrier to the early adoption of VoIP applications?

A significant barrier to the early adoption of VoIP applications was that both parties needed to have their computers running VocalTec software. During the early stages of VoIP technology, VocalTec was among the first companies to offer internet telephony services, but the reliance on specific software meant that both users had to utilize the same application for the calls to occur. This created a limitation in accessibility since users could not communicate with anyone who didn't have the same software installed, which restricted the network effect and overall usability of the service.

In contrast, the other options highlight various challenges that may have been present but were not as central to the initial adoption issue. Although high costs of broadband subscriptions and lack of universal compatibility with mobile devices played a role in limiting the growth of internet telephony, the immediate obstacle was the requirement for both parties to use the same specific software to communicate effectively. Similarly, the mandatory use of specific hardware did not apply as broadly since much of the early VoIP communication could occur using standard computers with the right software.