Product named Internet Phone, developed by VocalTec, was a pioneering product from two aspects: (1) It had the ability to do VoIP call over the bandwidth-starved public Internet of 1995, and (2) The developers of Internet Phone had to overcome the limitations of the hardware of that time which comprised of 486 based PCs and modems of 28.8 kbps speed.
Unique contribution here was to achieve real-time voice, including compression, with the standard PC hardware of 1995, and enable users to talk over public Internet in real-time. That was a major breakthrough that VocalTec achieved. Apart from Internet Phone, the first ever IP-PSTN breakout gateway was also introduced by VocalTec. The company demonstrated the first gateway at CT Expo in Los Angeles in the March of 1996.
There is no other company that can be associated with pioneering real-time voice over Internet. VocalTec was the first in introducing PC to PC VoIP client and hence is selected as one of the 10 organizations that established VoIP. There were other competing companies that were working on such projects who introduced similar solutions subsequently. These included Nuera, Net2phone, Cisco, Clarent, Vienna Systems etc.