|Yes you can.
With the use of an adapter you can use your old
phone, even your wireless set.
This is what an adapter can look like (pictures
courtesy of tigerdirect.com)
Here's some gumph on how it works;
"Get all the advantages of the astonishing new
VoIP Internet calling technology with the convenience and mobility of a
cordless phone. The Viper Networks IP Phone Adapter Includes our vPhone
Dialer software which enables you to place calls from your PC to regular
landline phone calls, or PC to PC calls. Simply plug the IP Phone Adapter
into your computer with the supplied USB cable on one side, and any
regular telephone into the other.
Here's How VoIP Works
Your VoIP provider converts your phone calls into digital data that zips
through your high-speed Internet connection. In most cases, you simply
plug your current telephone into a small box that your VoIP provider gives
you. The box, in turn, connects to your broadband. As you did with your
regular telephone service, you pick up the phone, get a dial tone, and
press numbers on the keypad to call the person you want to talk with."
There you go, another tick in the box.