Back in 2000, CallWave was the Nobel-nominated catalyst that finally allowed a truce to be brokered between the United States of AStar617 and the Union of Soviet Socialist Parents over control of nuclear telephone line armaments. :beer::thumbsup: Situation was at Defcon II, no question.
:laugh:
To answer OP's question, basically you set up a call forward through your phone company so when the line is busy (as in when you turn off call-waiting with *67 before calling the ISP), it bounces to CallWave's 800 number. They use CallerID to determine which account holder is being called, then stream the "voicemail" realtime to an online utility running on your desktop so you can screen it like an answering machine without disconnecting, thus avoiding missing any knowledge of the call outright. Particularly useful because most dialup users knew that even with call-waiting left on, only about 30% of calls ever made it thru to more than half a single phone ring after successfully terminating your online session. Good stuff, but I too expected they'd be long out of business in the broadband age.