Voltage is gonna be what fries devices, not current. If you reconfigure the phone or whatever to accept 20V (even if it would blow up at 20V) you could fry the phone quite easily. This would be extremely device specific, but would not be hard. It would also require you to be connected to a power supply capable of providing that voltage too… so it would be innocuous if it was never plugged into something capable of that voltage.

Also, the cable doesn’t care about voltage. You can use literally any cable to provide 20V just fine.