There is no standard in older phones about what to signal they are on a plug in charger rather than a computer. You would need to find what resistors the phone is looking for on the data wires, and install them in the socket of the USB charger. Just 5v and ground is not enough for loads of things to charge.
You may be able to find a USB charging cable for your phone that has them built into it. I found one for a motorola razr to get it to charge. Otherwise they will only charge off a PC if you have drivers and stuff and not off a dead battery either.