When the number is available, it calls you back. Wait for the message to tell you that the service is active, and hang up. You will be called back if the line becomes available within 30 minutes. Note that it automatically becomes deactivated after 30 minutes. To switch back to your other call, just press the button for another second.
Some phones have a flash button that does the same thing.
This may help you stay connected to the internet if you use a modem on that phone line. Then just press the receiver button again, or the flash button, and all three parties will be in the conversation. If you hang up, however, both of the other parties will be disconnected. This is automatic. If you have the full version of call forwarding, you can activate it to have all calls go to another number.