If you know someone who has a Beocom 5 (or even a BC6000 Mk2) it could be worth trying to reset the base with their phone. It could be worth buying a second hand BC6000 Mk2 – in my experience they are better phones than the Mk1.
Also worth asking a B&O dealer for a quote – if they still have their Beocom test equipment it shouldn’t take long to re-flash the SW of the Phone and/or Base.
Also, reading back through the thread I don’t think we actually tried a reset of the Beocom 5 itself. I understand that can also be done using the 9#9#9*INT sequence, as discussed here: https://archivedforum.beoworld.org/forums/t/35637.aspx