Dial 56700 from your BSNL mobile
Follow the IVR instructions to deactivate the caller tune
Send an SMS with STOP to 56700
Use the BSNL Selfcare app or BSNL Tunes app and turn off the caller tune service
Contact BSNL customer care at 1503 or 1800-180-1503 and request deactivation
Wait for the confirmation SMS after deactivation