Many modern phones use MicroUSB connectors can be charged from a USB connector. Is that what you want? If so, you will need a regulator to drop the automotive battery voltage down to 5V, which is the standard USB port power rail voltage.
Since cellphones can draw up to 1A when charging from USB, you will need a regulator that can supply 1A, or 2A if you want to charge two phones simultaneously. This regulator will need to dissipate significant power, because it is dropping 7~9V at 1~2A. For this reason, you should use a switching regulator, which is much more efficient than a simple linear regulator.
You also need to consider an issue known as automotive load dump - see
https://en.wikipedia.org/wiki/Load_dump. Usually, regulators for automotive use are rated for up to 70~80V input voltage, to handle this problem.
It's also possible to use a surge suppression device such as a varistor to limit the voltage coming into the regulator during load dump.
Maxim make two nice looking switching regulator ICs that would be suitable, but you would have to build up a circuit board. Alternatively, you could try to find preassembled power supply boards on eBay that use these ICs, or that at least have a high maximum input voltage limit. The Maxim ICs are MAX5035 (1A output current, see
http://www.maximintegrated.com/datasheet/index.mvp/id/3991) and MAX5090 (2A output current, see
http://www.maximintegrated.com/datasheet/index.mvp/id/5027).
You could use a module like this one:
http://www.ebay.com/itm/DC-DC-Step-...203?pt=LH_DefaultDomain_0&hash=item4166ffdbbb
... but you should add a varistor across the input to protect it from load dump. Something like this:
http://www.digikey.com/product-search/en?vendor=0&keywords=V18ZA40
Edit: You'll also need a USB type A receptacle. See
http://www.digikey.com/product-sear...nects/usb-dvi-hdmi-connectors/1442532?stock=1 and use the filter columns.