Hi Ania,
Yes you should be able to get it back.
It is potentially theft. Talk to the police about that.
Also, put it in writing. That is, put in writing and give to the keeper of the phone, your demand for its return, advising of the consequences of its non return i.e. a police report and or legal action. Not that you would necessarily begin legal action for a phone to be returned, but you can certainly say you will, as long as it is at least a possibility. Let us know how you go.