In order for messages to be sent as part of workflow actions, recipient records must be enabled.
Enable Records for Workflows Individually
1. Open the record for editing - see supported record types here.
2. Go to the Custom tab.
3. Check the Send Workflow SMS box.
4. Repeat for each record - alternatively, refer to Enable Records for Workflows in Bulk.
Enable Records for Workflows in Bulk
To enable multiple records for workflow sending at once, run a mass update on the records to check the Send Workflow SMS box. For instructions on running mass updates, see this NetSuite article.