[matrix2003]

Is that for power deliver, or signaling?

[p_l]

-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.

[amanaplanacanal]

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

[Kirby64]

Both. It powers old school telephones.