Issues on technology

Please someone give me solution for this problem !

In my OnePlus Mobile’s 1st sim slot not accepting 2g calls , but csfb happens while I’m initiate the call from LTE .

But same sim card putting in the 2nd slot is no problems came for 2g

And volte calls are fine and other Sim cards like idea is fine in the same slot , I’m using a idea sim card for checking the slot for 2g it’s fine !
The exact problem is my first sim slot is not accept or deliver the 2g calls from Airtel