You can also try the Talkatone or Whistle apps. They are good for free calling - well, at least within the USA (I don't know about elsewhere). Both are free apps to get from the App Store. With Talkatone, you get ad pictures and a choice to upgrade to a $19.99/year (USD) ad-free, enhanced service.
With Whistle, you have to listen to a 20 second or so commercial before you can dial (or, sometimes you just see ads, not hear them). I think there's an option to set up an account so you can pay to not hear the commercials, but I've not done it, so I don't know the cost...
Both have worked well for me in the past. Of course, and as always, YMMV.
Marilyn