Iirc the article mentioned that 555 numbers used to be reserved for internal use, so my hypothesis is that implementing external 555 numbers likely required som

Iirc the article mentioned that 555 numbers used to be reserved for internal use, so my hypothesis is that implementing external 555 numbers likely required some sort of retrofit. But I don’t have any domain knowledge here, so this is really just a guess ¯\_(ツ)_/¯