Into the 1960s my grandmother didn’t have a dial phone. You just picked it up and told the operator the number that you wanted to be connected with. She had a 3 digit number. My other grandmother was on a party line, with a unique ring pattern for each number on the line.
Phone numbers back then were handed out in the order a person became a customer. When I was a kid my parents bought a business and kept 'the old phone number' which was 62. Your grandmother could have been one of the first 999 customers
Was there a 'BR' in front of the numbers?
My grandparents lived in a small east Texas town. To call some one you would dial 5 numbers. My grandfather would not spend more than 2 minutes on the phone because “central was listening”. He lived from 1903 to 1981.