Samsung Galaxy S6 won't accept SIM PIN unlock code

[Android Central Question]

Recently my phone was working perfectly, and suddenly it was turned off in my pocket. When I turned it back on, it asked me for the SIM unlock code. I entered it correctly but it refuses to accept it. I tried with another SIM card and it's the same story: it asks for the PIN and refuses it, even when entered correctly. I restored the phone to factory defaults, but it's the same thing.

Any ideas about what can I do?

Thanks in advance.