You lost your phone
Yes bro, when you insert sim in any device (phone or data card) IMEI number is sent to that mobile operator.
So, if you file a complain, the police can trace your phone. Btw, it'll happen only when anyone inserts a Sim card.
For example, let's say your phone is with me and I removed your sim. After that I inserted one of my Sim (Say, Airtel). So, the IMEI of the phone will be sent to Airtel. Airtel, would be able to know the IMEI and all the details of the phone. If police has shared the IMEI with telecom operators. Then the telecom operators will send the details of that Sim card holder (Name, address etc.) After that police can go to that address and catch the thief.
Btw bro, Police are too lazy to find stolen phone. And there's 90% chance you won't get it back.