>>has anyone used VFP with a softphone application? What our customer wants to do is to have a button on the screen that automatically calls the client when clicked.

I did that several years ago using a Dialogic telephony board. That was before VOIP was as ubiquitous as it is now and used a phone jack.
I'm sure there are better VOIP solutions now.
The programming was fairly simple. The board came with an ActiveX control that exposed all the methods and properties you'd need.