I was told that, in ancient times, the monolith that was ATT designated the 555 prefix for it’s internal use - therby ensuring that no ‘real’ phone would ever be assigned a 555 prefix.
And your ‘area code’ startd life (1947) as a NPA - National Prefix Area.
I once was assigned 888-8888 - until the folks behind the counter (several of whom had had the number) assured me that I really didn’t want to hear from every drunk, every small child playing with a phone, etc.