Why do landline phones stay on during power outages? ?

Question

Technical references are welcomed and preferred!

Answer 1

They are backed up by banks of large batteries (at the telephone exchange switching office) that output -48 Vdc.  During power outage, the banks of batteries are charged by backup generators.

Answer 2

Landline phones do not run on mains power, but on batteries located at the exchange. Thus a DC voltage is supplied to your phone via the telephone wires. The batteries are kept charged by generators during blackouts.