I got to wondering what sort of social proliferation the telephone managed to achieve in England by 1919. Nothing exhaustive, but this is what I've found:

By the 1930s, it was common for affluent homes in the UK to have their own telephones, with networks spreading far enough for calls to be made across several cities. The majority of callers continued to use local phone boxes or pay phones until the 1950s and 60s, when improvements in home phone technology made systems cheaper and more easily available.

Ref: https://www.italktelecom.co.uk/blog/a-brief-history-of-the-home-telephone

1918

Leeds automatic telephone exchange was opened on 18 May in Basinghall Street - a Strowger-type manufactured and installed by the Automatic Telephone Manufacturing Company. It was the largest of its kind in Europe, equipped for 6,800 lines with an ultimate capacity of 15,000, and the first exchange in this country capable of being extended to give service to 100,000 subscribers. It was also the first in which the caller was required to dial five figures for every local call.

Ref: https://www.britishtelephones.com/histuk.htm

So for a cartoonist to be able to imagine having a personal phone at all in 1919, let alone a portable one, is pretty interesting. Maybe missed their calling as a sci-fi writer/illustrator :)