I have the 4 in 1 cable from Irwebonline.
To get the A300 to unlock with this cable you need to modify the A300 plug.
If you take the plug to bits you will see a yellow wire connected to pin1.With this wire is a thin black link wire that connects pin1 to pin 18.You need to cut this black wire.
You then need to cut the green (pin5) and red (Pin 4) wires where they come into the clip.
Then connect these wires together so you link Pins 5 and 4 together.
Its a bit fidly to do but if you have done it correctly the phone will light up when you connect the lead to it.
It will have no display and the red light on the phone will light.