What modern phone charges at 48V? I'm not aware of any that charge at even 20V, outside of a couple of gimmick devices. No Samsung or Apple phone charges at anywhere close to that, that's for sure.
12V isn't a 'standard' USB-PD voltage, so pretty much none of them. Tons of phones charge at 9V, though. And some use 15V.
The regulators/buck converters you speak of are inside the phones these days. They want a higher voltage even if the battery itself is 3-4.2V, so losses from cabling are lower and you don't need a special 5A cable to handle charging at the fastest rate.
Regardless, QuickCharge 3 is 20V and it's been in a lot of phones since it was introduced in 2016.