I can answer this. Don't ask me how- I am not a telephony geek! But I was determined...
First, I got Google Voice, and kept my number. I don't think that was an integral part of the process, but GV is cool.
Then, I got a free number from SipGate. It's a California number, because they don't have Florida numbers, but so what. They let you select a number from several numbers.
Then, I tried several sip client apps, is that the word? First I tried Groundwire. It looked _really_ cool, but I couldn't get it configured. Probably someone smarter could.
Then I tried Bria. Same story- I couldn't get it to work. I started to lose heart and almost gave up.
Then I tried WeePhone. Very basic. It was simple to set up and worked immediately!!
So I can make and receive calls on wifi only, with the SipGate number. And when I am on wifi but in airplane mode (I work underground with no ATT antenna) I get messages on my own cell phone number through GV, which sends me and e-mail, pops up a message, and if I am at my PC also pops up a message. ALSO when I go outside, I get a text! I realize I could turn some of those options off, but the novelty hasn't worn off yet. And GV plays the actual VM as a QT video, too.
Good luck- it was fairly frustrating but really only took a couple days of concentrated Googling and trying things to get it working.