Voltage requirements for charging Nokia phone
The old Nokia phones using a 2mm charging connector will refuse to charge if the supply is not current limited. I can't point you to any reference/datasheet but from my experience in building a home-brewed charger, you need to limit the supply to deliver around 100mA. I don't have my schematics handy right now but the circuit I used was based from LM317.



EDIT:

Here's the schematic for the charger I built (page 22 of the LM317 datasheet):


I replaced the 1ohm resistor with a 0.82ohms 5W resistor to get around 300mA of current (which falls within the limits in the Nokia datasheet). Also instead of 1.1Kohm, I used a 5Kohm trimmer so I can adjust the output voltage to 5.5V. Supply voltage is 12V so make sure to mount the LM317 on a decent heatsink. I have over-sized capacitors for input and output as I only have 2,200uF 25V available at that time.

Here's a photo of the kludge charger: