Reported my iPhone 6 lost, can the person who found it still use it?

I went to t-mobile to get a new phone because i lost mine, for the person who found my phone and has it can they still use it? T-mobile said they filed a claim, what does that mean?

Ask them. It's SIM may be turned off and it it's serial put into a list so if taken to another carrier it can't be re-activated.

Make sure you put it in lost mode also and put a msg on screen.

T-Mobile would disable it as far as it being a phone is concerned, the SIM in particular but everything else, if there was no password/code on it is pretty much fair game.

If there's any consolation if/when they try to reset it iCloud Activation Lock would prevent them from getting very far.