The problem with such a question is that the answer is so complicated. It's mostly the amount of current flowing into a vital organ that causes death.
If you run a current between two fingers on the same hand, only a small percent of the current will flow through, for instance, your heart. If you run the current from your left hand to your right hand, a larger percentage of the current will flow through the heart. A medical patient with, for instance electrodes for a heart monitor could have a much lower current threshold if there's some sort of electrical problem because .
Even though we say current is the problem, what we see most of the time is voltage. We are usually dealing with what we consider to be "constant voltage" sources. This would be something like a battery. It produces 1.5 Volts most of the time. If you have 0.01 mA flowing, it's 1.5 Volts. If you pull 500 mA out of it, it's still close to 1.5V.
Now, assume you have an exposed voltage of 50 V somewhere. If you walk up and touch it and have on shoes with rubber or plastic soles, the electrical resistance of your shoes is so high that you might only get a few micro amps. Change to leather soles, you still probably don't get much current. Now, assume you're touching a metal piece of furniture with one hand and touch the 50V circuit with the other. You get considerably more current. Now, consider if you have sweaty hands and are making really good contact with a grounded metal table. Now, assume you're standing in a decorative fountain with wet hands working on the water pump and you don't realize that the 50V DC power supply isn't turned off.
The threshold of "safe" voltage varies widely in these different situations because the electrical resistance varies so widely.
In the electrical engineer safety discussions, a "nightmare" scenario was something like "A technician is working on a piece of low voltage electronic equipment. The equipment has energized components with sharp edges, for instance voltage test pins. The technician slips and manages to spear a finger on each hand with a pin and pierce the skin. What's a safe voltage level here?" The answer was that there probably was no safe level.
With all that said, you mostly worry about voltages above 50V. You understand that lower voltages can still be dangerous in certain conditions. You become more concerned in wet conditions, or any kind of medical situation. Me, I start getting nervous above 12V, and ratchet up the nervousness as voltage gets higher.
You also understand that a lower voltage high current situation can cause thermal burn problems. For instance, shorting out a car battery with your class ring can cause a really nasty burn.
By the way, telephone wiring is around 48 Volts when the phone is on the hook, and we normally don't worry about that too much. However, if you're messing with some phone line connection and thoughtlessly decide to hold the connector in your mouth to free both hands, it can be an unpleasant or even fatal surprise. Especially if the phone rings, when the voltage jumps to around 90V.