When you add a new SMS recipient, the mobile number is sometimes formatted without country code. Then the system can add a country code that you can select. But if one always communicates with eg Norway, so it is easier if Norway is pre-selected whole the time. This is done in the respective publication settings. See pictures below.
1. Go to publication settings
2. Go to Settings
3. A bit down the page, below the fields for the preselected sender settings is the default country code.