Csfb happens but calls not connected ? How can I solve this?

According to my testing there is no problems in my phone and the sim but the problem I’m facing 2 more weeks ! If I make volte calls it’s fine ! And I removed the sim and checked another Mobile then 2g and 4g volte everything is fine ! When I remove the sim and put it in second sim slot then it’s fine !

1st sim slot only got issues while making csfb calls !
The phone is OnePlus 8 and the sim is Airtel

Someone please give me a solution !

Thanks in advance for your advice and suggestions