The major IM networks: AIM and Yahoo include voice chat capabilities. Skype also includes this capability. This means that you can "talk" with your user just like a phone but instead you use your computer - either a microphone/headset or the microphone built into your computer/laptop (if it has one). I decided to perform a test with a colleague - which one had the best voice quality.
To perform this test both myself and a colleague used IBM ThinkPad T42p's using the built in speaker and microphone. For the AIM and Yahoo tests, we were both on the corporate WAN - roughly 200 miles apart on the East coast. For the Skype test, my colleague was on the corporate WAN and I was at home, using a cable connection.
Both AIM and Yahoo had good sound quality. Not as good as a phone, but good nonetheless and certainly usable. The quality of Skype was noticeably better - extremely clear with excellent sound quality - even with the Thinkpad's built in microphone and speaker.
Now, if only Lotus Sametime had voice capabilities...