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Telephony has been using -48V as a line voltage for a very long time, probably > 100 years.

Is that for power deliver, or signaling?

Both. It powers old school telephones.

-48VDC was standard for powering telephone equipment, and is also common standard for direct current datacenter connections.

Signaling on POTS easily hit over 100V, btw.

I remember getting zapped when the phone rang when I was a teenager back in the 70's. Good times.