The voltage of telephone wire in The USA

I usually see about 48 volts Dc "On Hook", about 6-9 when "Off Hook", and about 90 volts AC when ringing.

The off-hook voltage is nominally correct, but it is actually "loop current" because the current is generally regulated. If memory serves me, somewhere between 25 and 50 mA.

Voltage on the lines, technically is negative.

I forget what I used when I put together a quick test jig.

They make a good point on the polarity.
Neither of the pair of wires are at ground potential. It depends on how you look at the Voltage. The Red wire is Negative with respect to Green. Green would be the positive.
I have seen TELCO trained people say they were told to reverse the red and black leads in their meter so when red was hooked to red, it would read as a positive voltage.
I prefer to place the red probe on Green, and the black probe on the Red to read the positive Voltage.
Ebaines and Kiss made a good point.

How much the voltage of telephone wire?

Same as it was on post #2 above assuming POTS or Plain Old telephone Service.

Voltage is on Post #1,
Current is on Post #2.

Is it possible to use these voltages for some devices to operate

... "on people?"

Pleas finish your sentence. Thanks.

People have done it, You may overload it, and cause the service to be shut off at the least, May be legal problems, and you wouldn't be able to use your phone.
It would be an expensive power supply(monthly phone bill).

May be around 40-70 volts, It will go up to 100 volts when it rings.