As the numeric telephone system grew, the initial 4-digit number sequence ran out of unique combinations. In the late 1920s, alphanumeric codes were assigned to the beginning of numbers to help identify geographic locations.
For instance, a phone number may be given as MElrose 4-2829. The first two letters of the exchange center name were capitalized to show which letters to translate to numbers. That meant someone would end up dialing 62, the numbers to match ME, before the personal line number 4-2829.