System configuration had a lot to do with it. Telephony was based on physical circuits. "Switchboard operators" literally plugged in cables to connect two phones together. Mechanical switches were just that -- mechanical systems which could join two lines. A telephone prefix (North American system) corresponded (generally) to a switch, which was a dedicated building, usually occupying much of a block, present in a given service area. If your town had a local phone-company building (or multiple buildings), that wasn't an office but the outer case around a large mechanical device. I was quite suprised the first time I was let inside one by the reality.

Virtual and digital switching, or better, packet-based networks, remove that problem. At the heart of most messaging systems now is the directory -- either of individual addresses, or of systems on which those addresses reside. Hence: DNS, MXs, and the like, for email.

(This makes me realise that one way around the Global Directory of Everyone problem of current messaging systems is to split up the address space again.)