Generally the cell phone requires a certain voltage to trigger the charger detection, these days something around 5V is gaining popularity as it allows for charging from USB.

An important consideration is that the charging source needs to be current limited as the internal charger in the phone will often use the current limit in the charger to dissipate power due to the limited power dissipation capabilities of the internal devices.

Other than that, 5 to 6V, 500mA current limit will get you into the right ballpark for most phones.