Ensure your BSNL SIM is 4G-enabled and your phone supports VoLTE
Insert the BSNL SIM in the primary SIM slot
Go to your phone’s Settings
Open Mobile Network or SIM & Network settings
Select your BSNL SIM
Turn on VoLTE or Enhanced 4G LTE Mode
Enable 4G/5G preferred network mode if available
Restart your phone
Make sure mobile data is turned on
Check that VoLTE appears in the status bar during calls
If VoLTE is not available, update your phone software
If needed, replace your old BSNL SIM with a 4G SIM from a BSNL customer service center
Contact BSNL customer care if VoLTE still does not activate