I recommend you try www.Flashphoner.com - there is everything you need.
You can downlad archive from main page and then you will find a client application (written to the flash, the code open, you can cnahge it)
For this solutions you'll need a server (any VPS for $ 20/mo. just fine) and then just write to support - firstname.lastname@example.org
with the theme «I want to introduce calls to the site» - and the guys will help you.
The solution costs about $1000, but if you really want implement calls to your webstie - to call yourself - it's worth it.
Depends on what you sell from the site, but the click2call average bayback in about 10-15 purchases.
This is about a couple of months. This direction is fresh, and people try it carefully. But within a year expecting the boom on this market - will all have laptops/netbooks/smartphones and a call from the site will be the norm.
Support there is very good, fast. They will help you install and configure everything without problems.
P.s. More will be needed several SIP accounts - take it from any voip operator
P.s.s. That's documentation for your programmists - http://docs.flashphoner.com