My thanks to everyone for all the above.
As I stated in my OP: "To the best of my scant knowledge of 'phones . . . "
I was (still am?) under the impression that with a twisted pair connection, there is no voltage across that pair - simply tones received or produced plus speech. That's why I expected it to be necessary to provide an external d.c. supply.
However, the post above from OscarFoxtrot seems to contradict that.
This 'phone does not have a battery backup.
Al.