Charger won't provide voltage it doesn't support. It won't negotiate it, and if it does then it is broken. If the phone tries to pull too much current, the charger won't provide it, and either shut down or limit the current.

If you took a cell phone that only accepts 9V or 12V and changed it to think it was permitted to accept 20V, you could easily fry a phone. Charger will happily supply it if you trick the phone side.

This would require malware on the phone itself, though. Possibly something quite low level.

Inside cables that support USB-PD, there are chips that tell the peripherals what level of power transmission they support, so that attack shouldn't be so easy.

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.