The SMSC stands for "SMS center", it is a system that manages the traffic of SMS (texts) in a mobile network. That's the place where the messages are kept for a while if they cannot be delivered instantly to the recipient.
Normally none of us has to be aware of it, because the SMSC is written to the SIM card by the mobile operator, so we don't have to configure anything.
On the file system of the SIM card, the SMS center is stored in EF SMSP (short message service parameters), the full path to the file is 7F10\6F42. The file can be read and updated, if you know PIN1.
The easiest way to change it is with SIM Explorer. It can display the file in a nice, human-readable form.
And if you prefer raw data - you can have that too:
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