found this from google.
There may be 2 different codes depending on how old the car /radio is.
Try 3963
and if your radio will not accept any digits higher than 6 ( ie 6666 is the highest )
then try 2211
failing that you will need to get your serial No, and use google to find an unlocker, or take it down to an electrical specialist.
if you google peugeot 307 radio code, the first result is a site I have used a couple of times for other cars, and its been ok.