OK, let's be technical.
You are correct, and I've said this all along, USB chargers ARE voltage converters. But, they are not "dual voltage", they are "multi-voltage"; that is, they accept any voltage (not just two voltages) between 100VAC and 240VAC. They probably accept voltages outside that range, too, but that is pretty much the range for the world (western Australia used to be 250VAC, but now, it looks like, that has been lowered to 240VAC. Japan is 100VAC (50 or 60 Hz), the US is nominally 120VAC, the EU is trying to standardize at 230VAC, some countries (Australia) are 240VAC. A USB "charger" will accept anything into which you plug it.
In the case of the product being promoted by Archer65, it converts 12VDC from a car battery to 5VDC for your phone. That's not unusual; many products are available for the same purpose. That product claims to be CE "tested and certified". Having previously done the work to put the CE symbol on a product, I can tell you that CE is a "self-certification". The manufacturer analyzes the safety of the product using guidelines specified by CE and then puts the CE symbol on the product. You must keep your work in a file that CE an see if there is ever a safely issue, but other than that, CE never sees or reviews your work. There is no such thing as CE testing and certification, like you would have with UL or CSA.
Dual voltage is an entirely different animal. It's primarily used with resistance heating devices, like flat irons, where a switch aligns elements that are designed for 110V to 125V to accept two (dual) voltages, in parallel for low voltage (110-125 V) input or in series for high voltage (220-250 V) input.
BTW, I did acquire online a USB converter with round pins so I can use it in Europe without an adapter (no worry about leaving the adapter in the wall or taping the two together).