Here's somethig from
http://www.v92.com
Modem-on-Hold
What will Modem-on-Hold do for me?
Many households use the same phone line for both voice calls and data (Internet), so when the user is browsing the Internet, an incoming call cannot get through. Modem-on-Hold allows you to receive an incoming call and stay connected to the Internet (Call-Waiting service from your phone company is all that is required). It also works in reverse; you can initiate a voice call while connected and keep the modem connection.
How much time will I have if I choose to take an incoming call?
The "hold" time is defined by your ISP. The V.92 specification allows for hold times to be anywhere from 10 seconds to infinite.
Do I have to redial to get back to the Internet?
No. When you hang up the phone you can resume browsing.
Can I stay on the Internet and make a phone call?
Yes. Initiating calls is easy with Modem-on-Hold. First, a Modem-on-Hold application is executed. This program suspends the data connection between your modem and the ISP so you can pick up your phone and make an outgoing call in the usual way. The application puts the modem "on-hold", flashes the hook, and a dial tone appears on the extension handset so you can make a call. When your call is complete, the modem will detect an extension on hook, flash the hook twice, and return to the data (Internet) connection.
How long will I have before the Internet connection is dropped?
Again, the hold time is determined by your ISP. For example, the Modem-on-Hold application can display the time allotted and also display a countdown so you will know how much time you have left for the phone call.