The Add-on selection page needs to be tweeked to block anyone without a voice plan from buying a long distance add-on. Several people have bought it only to discover it is unusable.
Either set up the add-on page to automatically block out the purchase option for those who are not on a talk plan... OR, start offering talk add-ons for those who wish to purchase limited minutes to suit their needs.
Thanks for raising this issue @Luddite, definitely needs to be addressed. I know it's a bit outside of the scope of this idea, but it would be awesome to have the option to buy limited minutes for data-only plans as mentioned by @imm1304
Upvoting, but as much or more for the comments (allow addon use without plan) as the original post. Agree that if you can't use something, it should be blocked for purchase though.