Mobile data not available during call in iPhone SE 2020, even volte is on

You will need to go to: Settings> Cellular>Enable LTE>Voice and Data. Make sure there is a check next to this category. Restart the device and test it out

The cellular-data options on your iPhone might vary based on your carrier's network.

If your carrier supports VoLTE

On your iPhone, go to Settings > Cellular > Cellular Data Options and tap Enable LTE or Settings > Mobile Data and tap Enable LTE. If your carrier supports Voice over LTE (VoLTE), you'll see these options:

• Off: Turns off LTE
• Voice & Data: Allows voice calls and cellular-data use over LTE 
• Data Only: Allows cellular-data use, but not voice calls over LTE

That's a carrier feature. Contact your cellular carrier.

Contact Airtel.