Check that your BSNL SIM is active and not under any lock or outstanding issue
Make sure your mobile number is eligible for porting
Send an SMS from your BSNL number to 1900 in this format: PORT
You will receive a UPC code and its expiry date by SMS
Keep your Aadhaar or other valid ID proof ready
Visit the nearest Airtel store or Airtel retailer
Request BSNL to Airtel mobile number portability
Submit the UPC code, your ID proof, and a passport-size photo if asked
Fill out the porting request form and complete KYC verification
Choose an Airtel prepaid or postpaid plan
Insert the new Airtel SIM when instructed
Wait for the porting to complete, usually within a few days
Keep your BSNL SIM active until the porting is fully completed
After activation, restart your phone and verify Airtel network service
If needed, complete any final SIM activation steps as guided by Airtel