Thanks All (well, two)
I think head office number is the most likely.
My own experience is that in the less olden times (70s and 80s). If you moved area, you had to change number.
In the late 80s / early 90s my ex employer Stirling Microsystems moved temporarily from 231 to 241 Baker Street (The more astute among you will notice I worked a few doors down from Uncle Sherlock).
The move was across a BT boundary?.. and the boss man didn?t fancy the cost and disruption of changing the number(s)?.
After 231 was refurbished and it was time to move back the BT salesman asked where they should collect the old equipment??
The MD said ?You see that piece of telecom wire going across the pub garden?.. it?s at the other end of that?