A PC-to-PC Internet phone is a remnant of early VOIP offerings. Many people still use this type of VOIP service for one reason -- in many cases, it's still free, sort of. PC-to-PC Internet phone service operates off a software program. To use it, the software must be installed on your computer, and the computer must be turned on and signed on to the Internet. Then, with a microphone headset or a microphone, installed sound card, and speakers, the computer is actually the device with which you communicate. Many of the software providers that offer this service still offer it for free, or at a very affordable cost. However, as the figure below shows, the service only works with other computers that have the same software installed, and to be used, the receiving computer must also be on and signed on to the Internet.

Even with those downfalls, PC-to-PC Internet phone service is hanging on. With high-speed Internet access at an all-time high, the quality of the calls has improved dramatically from its early days in the VOIP arena.
For many people, the price (usually free) is perfect. Still, if you're planning to use your VOIP regularly, the inconveniences of being tied to the computer and limited to only those who have the same software might make this a poor selection for your needs.

A variant of this is the voice support with MSN instant messaging, usually effected by a microphone in a webcam.