If you have a wifi connection you can make and receive free calls to USA and canada with the Google Hangouts app.  Just get a Google voice number and download the Hangouts app from the Apple app store.  You then sign in with the Google account that is same as your Google voice account and you will be able to make calls.  To receive calls you have to go into Google Hangouts app settings and turn on “Ring calls on hangouts” and that will ring your iPhone or iPad or iPod when you are called through your Google voice number.  It will ring into your Hangouts.  You can combine this with the free 200meg data plan from T-Mobile and you may never need a carrier ever again. Google hangouts app iOS