The first two letters in an exchange equated to the first two numbers of the exchange number. Early on, if you were in the same exchange, you only needed to dial the last five digits, like you do today in an office.
Each telephone office exchange had a two digit number assigned, and a nemonic was created to help remember it. Where I grew up, my exchange was “Prescott 2,” with the first two letters PR equating to “77.” Thus my phone number started with “772.” This is why there are letters on your phone dial.
Apparently, the phone company figured people could remember a name and five digits easier than remembering seven digits.
It was that way in my hometown until the late 1980s.