When I was a kid '555' was my whole [UK local] phone number.

I remember when they added 0 to town codes...but just 3 digit?

Yup, it was a local exchange {for local people!?}. The story I heard was that the owner of the manor where the exchange was had been a telephone pioneer.

It still is, for every area code I believe. It's the same as calling 411 where I am. (Information)