Kakao Talk is a free app which allows you to make phone calls using WiFi only, with no cell phone plan or bill.
My daughter is teaching in Korea. She bought a used cell phone for $10.00 when she first arrived and that has been her only phoning expense in the last year.
Just about every teacher there, and other people her age, are familiar with this app and other apps like it. This could easily make phone bills obsolete in the city, where WiFi is everywhere.
It can also be used by someone with a phone plan, when there's no WiFi. My data plan you has plenty of space.
Along with voice, you can also send text and photos, so the functionality is just about the same as with any other phone system. I have used Skype, and there always seems to be a problem with the quality of WiFi or various other issues. This system has given very little trouble.