Yes, you can change the SIM-Card anytime. The.Blue.Box-II has no SIM-Lock and you can use every GSM-Network.
Befor, you have to turn-off the PIN-request. Therefor you put the SIM-Card in a mobile. There are no data on the SIM-Card. As a result, you have to nothing to change in the configuration.

The.Blue.Box-II whistles whenever she is not ready for operation.
It may have failed, the GSM cell. This can be checked using a mobile phone with the same network.
The “A” means “book in”. The.Blue.Box-II try to book in.
Check that no SMS is stored on the SIM card, which The.Blue.Box-II can not delete. Put the SIM card into a mobile phone and delete all SMS. You should check the function of the SIM card by sending a SMS.

Every day makes The.Blue.Box II a “reset”. This is necessary for safe operation.