As part of the Smart Timesheets feature you can have an automatic reminder sent to your technicians to review and submit their timesheets for the week.
In Reports - Timesheets - weekly tab click on settings on the right hand side to bring up the Timesheet settings. Select the start day for your timesheets and check the Send SMS reminder to technicians to submit timesheets. The SMS will be send the at the selected time with a link for the technicians to submit their timesheet. Click save to save the settings and close to settings popup.