I found the answer to this question in “The Small Times” coffee companion paper. I’m going to quote it word for word so I thought if I mention “The Small Times” at least three times in this article they won’t mind me using their content. Thanks “The Small Times”
Because instead of getting its power from the national grid, your telephone gets its juice straight down the line from Telkom. The fixed-line monopoly has its own so-called Uniterrupted Power Supply which runs off giant batteries. The batteries are kept fully-charged using Eskom power, but Telkom also has it’s own back-up petrol and diesel generators in case the grid goes down for any period.
The current is carried down one of the two wires connecting your telephone to the network. The other carries your voice, obviously.
A telephone can’t run off your metered supply because it needs only a fraction of the current pushed out by a 15 amp wall plug.
Only about 20 to 30 milliamps are needed to enable the microphone in the mouthpiece to modulate your voice, and make the thing ring.
Which is why you can phone while you’re in the bath, and live to towel yourself down.