Can i use an old SIM card in an unlocked cell phone?

My boyfriend got a new phone for work. the old phone he doesnt use has the SIM card in it. The tech guy at work did not deactivate that SIM card, they just got him a new phone and activated it with that number. I have an unlocked phone. Can I put the old SIM card in the new unlocked phone and make calls? it would show in the account for the new phone he has?