TTY phone fraud and abuse

Yes. Depending on the countries and telcos involved, this is either
illegal or "irregular" network access. Other schemes involve call-back
(with the Internet as the signaling channel -- I first heard of that
being used in 1994, when most people outside our business had never
heard of the Internet) or calling through a third country if the
difference in rates makes that profitable.

    --Steve Bellovin,