What you need

New gear to add

1. A Soft Phone account that uses the SIP protocol.

You can get a free or inexpensive soft phone service from one of many providers. Learn more about free SIP phased softphone service at http://snapvoip.blogspot.com/2008/06/free-sip-account-provider-list-free-sip.html. In my case I added a paid Vonage Soft Phone line for $9.99 per month

2. Buy a voice gateway Linksys SPA3102 for about $70. See http://www.cisco.com/en/US/products/ps10027/index.html

Note that it is also sold as Cisco SPA3102 and Sipura SPA3102
(I bought mine at the College St, Toronto location of Canada Computers)