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These days companies like Vodafone, Jio & Airtel that charge you something around Rs. 199 a month and give you free 1.5gb data, unlimited calls and sms. Have you activated any of these packs?
Or are you using the credit balance in your phone to send an sms?
The reason I ask is that if you have 0 credit balance or you dont have a monthly pack activated then the SMS will not go through. Or if you are a post paid customer and your bills haven't been settled then all your services get deactivated.
I had this issue with my secondary number and I thought my Mi Max 2 was at fault. Turns out the balance of my secondary number was -14 Rs! Which meant I could not send any sms's before paying that 14 rupees to Vodafone. Once that was done my SMS service went back to normal.
So just make sure that there is no problem from your service provider that is disrupting your SMS service.