Check whether the SMSC (Short Message Service Center) number the phone uses is correct:
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Dial
*#*#4636#*#*… “Testing” menu appears. -
Choose “Phone information”.
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Scroll down to the “SMSC:” input field.
It’s not very intuitive territory there. This text field can be empty, and the SMSC the phone uses can be fine nonetheless.
First use the “REFRESH” button, this will put the SMSC the phone currently uses into the field, or “refresh error” if there is none.
After the phone number of the SMSC there might be an added number given, separated by a comma. This is harmless.
You can check whether it is the correct SMSC by looking it up online for your provider or by asking your provider for the correct number.If you decide to enter a different number here, you will have to confirm your input with the “UPDATE” button next to the field, else it will not be adopted by the phone.
If your SMSC turns out to be correctly entered, try whether it helps to turn off the phone (no reboot, real shutdown), wait a few seconds and turn the phone on again.
In case you use a physical SIM card for SMS, not an eSIM, you might use the opportunity to reseat the SIM card when the phone is off … can’t hurt.
For a more extreme measure there’s a network reset in Settings - System - Reset options. Be aware that this will affect Wi-Fi and Bluetooth, too, so be prepared for the loss of e.g. Wi-Fi passwords and paired Bluetooth devices.