According to
Wikipedia, ETL was a joint venture between LM Ericsson of Sweden and the National Telephone Company, formed in 1903 and based in Beeston, Nottingham. In 1912 it became wholly owned by LM Ericsson but they were forced to give up majority ownership during the second world war, as the factory was used for war work and there were security concerns about information being passed to Germany via the neutral Swedes. By 1948 it was an independent company and remained so until 1961 when it was taken over by Plessey. In the late 80's, it became part of a GPT, a joint venture between GEC and Plessey, then ended up as part of Siemens. A bit off topic but I worked for GPT when I first left university and remember going to meetings at the Beeston site, so was interested to see what had become of it.
If I remember correctly, magneto phones were mainly used for point-to-point links where there was no telephone exchange eg army field telephones. Each phone had its own battery and as John Webb says, the magneto handle was cranked to provide a ringing current to call the other end.
This
advert says that they supply"British Dominion and Foreign Governments and the principal Railway and Telephone Companies of the world" so perhaps it was for export rather than UK use?
There are some more gloriously evocative adverts
here - I especially like
"When the 'phone stands between life and death"!