My HTC A9 phone is asking me for an 8-digits password only when a SIM is in the device. I already tried it with the PUK of the SIM what does not work. If there is no SIM in the device, there is no password prompt and I can use the phone normally. With no SIM in the device, I did a factory reset of the phone, but nothing changed.

The password prompt says: "Network locked". Is this a SIM-locked phone?

What is going on here?