Every smartphone has a unique IMEI number assigned to it. IMEI numbers are found on the box of the cell phone. Check the box if you still have it. Otherwise you can find out your IMEI number from the phone website or customer service.
You will need to launch a FIR with the police, attaching a copy of the IMEI number with it. Then give a copy of this to your service provider who can track the phone based on its unique ID number and meanwhile block the handset so that it cannot be used by anyone else. IMEI number helps to tracks the handset, even when the SIM is changed or the SIM card is not activated. Once the phone is traced, the police should be able to retrieve it