Looked this up after watching Better Call Saul and noticing a phone number in one of the episodes!
Once upon a time, the 555 exchange wasn’t assigned to real telephone subscribers. So it was it was part of fictional phone numbers that wouldn’t annoy users. The Verizon thing must have come later.
Directory assistance is available at 1–555-1212.
Not sure why they would open these numbers up for access anyway, a lot of PABXes are programmed to recognize the 555 prefix as bogus and just drop the call without even attempting to dial it over national lines
I’ve always hated 555 numbers, because they immediately dispel the illusion. They should just use invalid numbers that have an area code that starts with 1 or, better yet, an exchange code that starts with 1.