To track a lost mobile phone with IMEI number, you must first report the lost mobile case to the police. Thereafter, you must contact your service provider to commence the procedure of issuing a duplicate SIM card. You can also visit the official website of CEIR (Central Equipment Identity Register) - https://ceir.gov.in/.

In order to track a lost cell phone using the IMEI number, you can follow the steps mentioned below:
1. Open the Play Store on your phone and search for ‘IMEI tracker’. You will find several results.
2. Install any IMEI tracker with good ratings.
3. Launch the application once it is installed.

I hope this helps. Let me know if you have any other questions.