How to solve sms error 21 in airtel, ideal, reliance, vodafone and jio

I have seen that some of guys reporting that they can't send message from their mobile. While trying to send sms they got a error saying error 21.
I am not sure that this method will work for all of you, but you should try this once.
Try this step by step:-

1. Go to phone's dialer and dial *#*#4636#*#*

2. This will open a new window. Here tap on phone information.

2. Now scroll down for SMSC.

4. Here tap on refresh.

5. Now your SMS service center number will appears , just check them if they are ok, than tap on update.

That's it. You have done. I hope this will fix your problem. If still you are facing problem let me know in comments section i will help you.