1. Open a list (from Marketing > SMS > Contacts, tab Lists) and click Add CSV.
2. In Import SMS Contacts, click Download template to get the expected file: two columns, contact and name, separated by a semicolon (;), with the first row as the header (it is skipped on import).
3. Fill the file, then click the upload area (".csv files only") and pick it.
4. Leave Add country code (+X) to contacts? on if your numbers are written without a country code — Sprwt prefixes your account's default country code; turn it off if the file already contains full international numbers.
5. Click Import Contacts and confirm the prompt "Do you still want to proceed with importing these contacts?" A static reminder above the button already warns: "Only import contacts who have opted in to receive SMS messages. Sending unsolicited messages may violate privacy laws."

For each row Sprwt reuses an existing contact with the same number or creates a new one (Receive SMS on, name defaulting to the number if blank) and adds it to the list. Errors: "Unknown File" (nothing uploaded) and "File is not CSV" (wrong extension).