No, car phone chargers use a DC to DC converter, mainly because that's cheaper and smaller. It has the added benefit of being much more efficient than a DC to AC to DC scheme would be. Also, they cut down to a few milliamps or even micro-amps once the phone battery is full.
Typical cell phone batteries are 3.6 volts. Usually, the cell phone needs a 4.5V to 5V source to charge the battery from.
The simple and cheap way to do this (from a 12V car battery) is a linear regulator. It wastes over half the power used as heat, but the power used is so low that it doesn't matter too much. It also consumes almost no power once the battery is charged.
The better way to do it is with a switching regulator. That's only slightly more expensive, and it wastes less than twenty percent of the power as heat. These also draw almost no power once the phone is charged.
The only phone charging scheme that I know of that would use an AC converter would be where someone would use a 12V to 120VAC inverter and then plug a 120V cell phone charger into that. I am assuming that this is not at all what the OP was talking about.
If you can point to an actual cell phone car charger that uses an AC converter, please provide a link to it.