Just had some emails from Leon, very quick reply.
He told me to try to rotate the button, and since it doesn't work, he told me to try to change it.
I will buy the correct key tomorrow (any reference? torks?), if it doesn't work ISC will take all the charges for labor/materials/shipping.

I will let you know...thanks andy for the picture.
any reference for the correct key? any specific procedure to follow to dismantle the handset cover?