I recently ported several mobiles from EE to BT Mobile. I wanted to set a SIM PIN in one of them and attempted to change from the default (5555) to a five-digit number. On restarting the phone an error message to the effect that this was an incorrect PIN was seen and the default was not accepted either. Several iteration of this behaviour involving repeated application of the PUK code and entries of new PINs together with changing lock and unlock settings in the handset led me to conclude that only a four-digit PIN is accepted by a BT Mobile SIM.
I note that BT recommends that the default PIN is changed to a four-digit number but in my opinion that needs a little clarification (if indeed my experience is typical) and doesn't give as much flexibility as it might.
Is anyone using a PIN with anything other than four digits for a BT Mobile SIM?