From what I've heard:
110 V (or 220 V; household voltage pretty much) is dangerous (i.e. can kill you) I think there's consensus on this, no need to try 🙂
60 V (old telephone lines) is supposedly dangerous (never tried, only heard it once… probably won't try)
From what I know first-hand:
9 V is not dangerous (I've put a 9-V battery on my tongue, nbd… actually it kinda hurt!)
1.5 V can indeed be quite shocking with enough current (fell for one of those "Do you want some gum?" tricks back in high school…), but they sometimes do not use 1.5 V with the low amperage levels, some use a DC motor to vibrate and complete the trick.
So I guess there's two parameters here, voltage and current… but are there rough numbers on how much of each (or in combination, which I guess would be power) would be considered hazardous?
No old telephone lines have always been 48vDC well at least since from 1950s, if your skin is wet you can feel it slightly, like on your forearm. Now the ring voltage is 90-110vAC with a 2 on 4 sec off cycle (USA). It will ring your bell but good, should you be touching the wires when someone calls. The ring voltage rides on top of the 48vDC, so its present on the same two conductors that the voice voltage(DC) is on. Luckily it's 4 seconds off will give you a chance to get off the conductors with a scream (of pain).