Are phones marketed by MVNO operators locked to the underlying network only or is there another level of locking that would predent the phone from working with a SIM from the underlying network.

For example, if I buy a phone from a T-mobile based MVNO,

• will the phone work with both SIMs- MVNO branded SIM and T-mobile SIM?
• or will it work with only MVNO SIM?
• or the MVNO sells the phone with a T-mobile SIM?

Thanks guys.