I finally worked it out.

In some of the early reviews of my book, people referred to modern touches and the introduction of a telephone. I was confused by this as I did no such thing.

I only figured it out when I was listening to the audio book recordings. I believe the telephone thing has sprung from The Drury Lane Pawnbroker. Towards the end Holmes mentions having made a few calls.

People have got the wrong end of the stick. I meant this in the sense of physically calling on (visiting) people..

E.g. I called upon my friend...

Sorry, no telephones, but I understand the confusion.