First, why do you want to reduce the current? What do you want to acheive?
The charger is really just a 5V power supply, not a real charger in itself. The charger circuit is built into the mobile.
Power supplies like that provide a voltage, it is the mobile that will try to draw whatever current it needs. The power supply does not really control the current, all it will do is (possibly) cut off if your mobile tries to draw more current than the power supply can safely provide. The mA rating on the power supply is the maximum that it can safely provide to a device.
So, all that matters then is that your 5V power supply can provide enough current for the mobile. To out how much that is, check what the mobile's manual says. Or, better, put an ammeter in line with the power lead to measure the current drawn when charging an empty battery. That might be erratic though depending on what the mobile is doing.