Welcome to Android Central.
Is this a new device for you?
Has this issue always been this way, if not when did it start?
What device do you have?
What carrier do you have?
Was this a new device or an unlocked used device?
What app are you using for SMS?
Have you checked the app permissions to see if it is allowed to send you notifications, or have they been turned off?
The one thing I can tell you is some unlocked devices and some "unlocked" devices that were on different carriers than the buyer is trying to activate them on, will some times require contacting the carrier to fix this issue. I had this issue with a new unlocked device I was BYOP on Verizon and it required contacting them before I was able to receive messages.
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