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 comparing it with the number your provider gave you.
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 you use two SIM cards, be aware that the setting here only affects SIM slot 1.
If you want to apply it to the second SIM, you would have to put your second SIM into slot 1 beforehand.
If your SMSC turns out to be correctly entered, there’s more to be found in this forum with the search function: