I looked into this and ordered a sim but I haven’t received it yet. The cost was .99 - I had to download an app and the entire operation appears to be based off the app. Talk and text are completely free and app based (so if you put this app on your normal phone you can have a second number/line on your normal phone for text and calls through the app). If you want to use an unlocked phone that does not have service currently, you don’t need to buy the sim for wifi use. If you want talk/text away from wifi you need to buy the sim but there are no other costs except the .99 cost of the sim - unless you want no ads, data (although I read that 2g is free but accounts using excessive 2g may be terminated), a monthly fee for certain non-free phone numbers, or to “lock in your phone number” (not sure what that really means yet as far as losing v locking in a number)…
I don’t think it will work with a flip phone but maybe I’m wrong - I don’t have an old one to try it with (I wish I did) and I’m going to use it with an old school iPhone SE (2016ish). The app, phone calling and text features already work fine when on wifi. When the sim arrives I’ll try the call and text features and see how it goes…
Btw so far the ads are simple unobtrusive banner style ads at the bottom of the screen…