[greenbit]

Yes, in the old systems, you'd get about 90 volts AC down the line to ring the mechanical bell ringer. Once saw a guy nearly fall off a ladder, splicing phone lines with bare hands. He thought the relatively low voltage was safe enough, but then someone rang him in the middle of the job.

[brewtide]

I know 30+ years ago as I kid I learned this in my parents basement as I was rigging something up.

It is more the surprise, as if one is ignorant to this fact it is not expected at all.

[RupertSalt]

I had to refresh my memory about the hybrid use of AC and DC current in telephone networks.

The Alternating Current signals could be used over longer distances and were effective at making the bells ring, moving the clapper back and forth. This back-and-forth is exactly what makes AC so deadly in the body, should it cross through your cardiac muscles, for example, and set the muscles twitching at 50 or 60 times per second.

[avianlyric]

There’s nothing inherently deadly about AC nor anything inherently safe about DC. If there’s enough voltage available to drive current through your body, then electricity is deadly regardless of if it’s AC or DC.

In general AC tends to be a little safer than DC, because the voltage is constantly reversing, which means it’s constantly passing through 0V, creating moments where you don’t have current driving through your body and forcing all your muscles to contract. Those 0V crossings create moments where you can let go of whatever is electrocuting you. DC on the other hand has no such 0 crossings, if there’s enough voltage there to drive current through you, then all your muscles will be stuck contracting until either the power is turned off, or until they’re all so fried they’re not physically capable of contracting anymore.