As part of the Smart Timesheets feature you can setup automatic reminders for technicians to review and submit their timesheets. 

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 weekly timesheets

Select if you want your technicians to submit timesheets daily or weekly

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 review and submit the timesheet for the week/day. Click save to save the settings and close to settings popup.