This
is a premium Yahoo! Messenger feature.| Signing Up for Phone Out | Placing a Phone Out Call | Putting a Call on Hold | Managing Your Phone Out Account |
When you have a Yahoo! Phone Out call in progress, you can place the call on hold, just as you would with a telephone that has a hold feature. While the call is on hold, the connection is maintained, but you and your contact can’t hear each other. Putting a call on hold allows you to place another call or to answer a call from someone else. You can leave a call on hold indefinitely.
In the Call window, click the Hold button.

A message informs you that the call is on hold, and the Hold button changes to a Resume button.

When you’re ready to resume the call, click the Resume button.

Tip: If your friend puts you on hold, you won’t see any change to your Call window, but you will hear complete and utter silence or maybe Musak.
When you complete your call, you can review your Yahoo! Phone Out account to see how many minutes you have left on your phone card.