Airtel — facing delay in receiving sms - otp, banking services etc

I am facing delay in receiving SMS such as OTP from bank or any rest activities since 3 months. I have already reported twice to Customer care via call, but they are always saying they will resolve the issue within 24 hours like that..but no reply or response from them yet.. :(

Please help me here, otherwise I need to port my service from Airtel to other providers. I was using Airtel since more than 6 years, so this is the first complaint I raising to Airtel and having bad immpression with customer care services also.