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I would like to share a solution to a problem I had.
I'm not sure when it started, but suddenly, I could not send any SMS from either Sim1 or SIM2 on my REDMI NOTE 2.
My solution after trying many, many other things which didn't work was:
1) Go to System Apps > Messaging > Additional Settings > Short Messaging Service Centre (SMSC).
2) Select your Sim slot 1 or 2.
3) Depending on the ur service provider, key in the SMS Service Centre Number. You can get this by contacting your service provider or just search for it in Google.
That's it. You probably have to wait a while. It may not be necessary, but I restarted my phone. I went back to the SMSes which could not be sent out (indicated with the red "failed" marker). I clicked on it & suddenly it could be sent out.