First, this code will only work for Nokia bluetooth equipped phones.
For the trick, when on standby mode press *#9990# and something will work silently on the background. I believe on Symbian phones it will display a “Bluetooth test mode”. For Series 40 it displays nothing, and will result in a bug – whenever you try to turn of the bluetooth it will refuse to.
We really don’t know it’s purpose but I believe it has something to do with the bluetooth configuration.
Thanks to Francis for the tip
Hope this helps 😉