On a normal phone line you get 50v dc at a low ampage, you can feel it, bit uncomfortable but thats all. if the line rings then thats 90v ac and you most definately feel it and it hurts!
ISDN runs at 95v, nowhere near leathal but gives an unexpected kick. DACS is the biggie at 140v. The biggest risk is when your hand comes back as an involitary action from the shock, Doing this on the frame in an exchange can result in cuts to the back of the hand as you catch the neigbouring fuses, known as frame rash.
HERES THE IMPORTANT BIT HENCE THE CAPITALS AND ANY ELECTRICIAN/ELECRTICAL ENGINEER WILL BACK ME UP ON THIS.
ALWAYS ASSUME THERE MAY BE MAINS/HIGH VOLTAGE ON ANY WIRING, INCLUDING TELEPHONY. I HAVE HAD QUITE A FEW JOBS WITH MAINS VOLTAGE ON THE LINE. TYPICALY ON THE INTERNAL WIRING. IT DOES HAPPEN
A BT ENGINEERS HAWK, 9083, BUTT PHONE ALL TEST FOR HIGH VOLTAGE FIRST FOR THIS REASON.
ALSO AS PREVIOUSLY STATED DO NOT TOUCH MAINS/TELEPHONY IN A THUNDERSTORM, SUCH SURGES DOWN THE LINE ARE COMMON. AND FOR SAFETYS SAKE I'LL REPEAT THAT, LIGHTNING SURGES DURING A STORM ARE COMMON! THATS THE ONE TIME BT ENGINEERS ARE NOT PERMITED TO TOUCH ANY OF THE NETWORK.