Phone app for PC

Does anyone know of an app that can be loaded onto Windows 7 that will act as another extension of my phones in my home?  I have COX phone and internet service installed. 

I have a "softphone" installed now (which I use with a headset), but am losing it due to job loss.  I find the softphone to be a great tool for making calls.

I see that COX has a "kit" that can be purchased, but I am looking for something much simpler: PC app and head-set.