It will depend on the make and model of the freeview box.
Normally this is very simple but you have forgotten to supply the details of the make and model.
Rather than get someone to list every solution for every freebox it would be easier for you to start at 0000 (or 0000 on some boxes) and try each number in succession freeview.
Failing that you could ask the question again but this time include the model and the manufacturer.
I suggest a title of something like
"PIN for Humax HDR-FOX T2" (but obviously substitute the mysterious manufacturer and model that you own) as that will make it easier for people to understand what you are after.
Neil S
August 2014
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Try fixfault.com to fix dtt receivers.