Samsung S5 weird "no service" problem

Hello everyone,

I just bought a second-hand phone, replaced its screen and started to work with it.

I was told that the locking of the phone is unknown. I bought the device from UK, i'm not living there.

When i putted my sim in, first it showed "no service" so i guessed it is locked but when i'm "playing" with the network options sometimes it recognizes my sim and there is a service and i can make calls!

When i'm making calls (i was succedded to talk with a friend about a hour) the service is not disconnecting but few seconds after the call ends it may show "no service" again.

i have no idea what its problem.

I would be glad of help with that issue.

Best Regards