I use these on my WIFI only ipad
NetTalk - was good until I found out it wouldnt always connect to MagicJack users or Vonage.
Whistle - works all the time and gives you a tele number so people can call you. When you call out you have to listen to a 20 second advertisement.
Nettalk, whistle and talkatone are free
Each has their pluses and minuses
Nettalk pro: works without ads or other requirements
Nettalk con: doesn't connect to all phone numbers for some reason (majicjack, vonage) - just keeps ringing
Whistle pro: so far connects to all numbers and allows conference calling
Whistle con: have to listen to 20 second commercial before it dials out
Talkatone pro: seems it just works and people like it
Talkatone con: need to have google voice account and need ios 4.x on iPad