Sebastien,
A Beocom 5 can be connected to BeoLine PSTN, BeoLine 1 or BeoLine 2 base units.
A base reset can ONLY be done using a phone connected to the Beoline base.
If you don't have at least one phone connected and working with the Beoline base you cannot reset the pin.
Do you have at least one phone connected and working with your base ?