Joliette, a quick google search looks like they do support Mac, they didn't back when I was looking at them.
I do like Skype, and you can call anywhere, the rates are great, and you can get a unlimited national plan.
Just remember with any VoIP server the call is only as good as your internet connection. I have used Vonage for years, and I now use VoicePulse.
I rarely have a problem.