Trunk subs
Trunk subs, or Trunk Subscribers to give them their proper name, are defined in BT's price list as exchange lines terminated on an exchange manual board. Clearly only special types of subscriber are entitled to this class of service, which is intended to give these subscribers a high level of priority.
Examples of past and present trunk sub users include:
Residences of the Royal Family;
Ocean liners in dock at Southampton;
Main BBC transmitter stations;
Former Regional Seats of Government;
Key electricity generating stations;
Coal mines;
Other public utilities;
Imperial Hotel and Winter Gardens, Blackpool, for press, radio and TV use during party political conferences;
The payphones at the Royal Welsh Showground (in pre-STD days these were Trunk Subs off Llandrindod Wells);
Permanent facilities rented by national newspapers at racecourses for facsimile machines used for transmitting photographs to their head offices in Fleet Street.
Telephones used for trunk subs were often red Teles. 706CB. The red handsets often seen fitted to black Teles. 332CB in pictures of World War II military control rooms may have indicated Trunk Subs as well.
A contact in BT adds:
Trunk subs were initially lines which connected directly to an appearance in the trunk multiple of a manual board (hence the name) and thus got a preferential operator treatment. I'm not sure if they existed before the introduction of automatic local exchanges—that was the point at which they became valuable because, apart from getting priority over ordinary subs, they called the operator directly without bothering with dialling 0 (since only local calls could be made automatically the fact that trunk sub lines could not dial was not usually a disadvantage).
As I understand they were provided to various high priority callers such as royalty and some military. As the automatic system became more capable and more reliable so that virtually all calls can be made automatically and usually more quickly and reliably than using an operator the benefit of trunk sub status must be negligible but I don't know for certain whether they still exist.
Another informant states:
Army SO2 Signals for Eastern District (based at Flagstaff House in the Colchester Garrison) had a trunk sub number. Last time I had occasion to look at the records (1992 when I was working in Colchester) it was still active. There were also trunk sub numbers in use for Essex Fire HQ in Chelmsford.