Patrick,
> I'm not sure if you can help me but I've not been able to find
> information on when "unlisted" phone numbers first came to use.
FWIW:
The New York Times reported on 6/16/1960 that they began charging for unlisted numbers (obviously available long before that). The phone book was thicker because many subscribers chose to be listed rather than pay the 50c per month fee (about $5.00 in today's money).
The same article reported that Direct Distance Dialing (1+) was available for all NYC subscribers. They could dial up to 39 states and provinces. Obviously many areas couldn't yet receive DDD calls in
1960.
The phone co switched from a full name BUtterfield to two letters only BU.
Also the phone co began to use two letters than had no English equivalent, e.g. TF and LF [sic]. Even though they could've found names for those combinations, they wanted to try the idea out.