When SMS released, it was I believe, known that certain prepaid plans would not work even back then. 10 years.
There have been mixed reports on whether it works with prepaid mobile phones, but where it hasn’t been working that’s usually because carriers don’t allow the receipt of SMS text messages.
Blizzard did not make these directly either. They were made by Vasco, which I believe goes by another company name. Not sure why Blizzard stopped carrying them though, or if Vasco stopped making them.