Drive offers the option to contact customers/clients by SMS. This will need to be set up in system settings first.
To send one, go to My Communications > SMS. The below screen will appear - any previously sent items will show here. You have an icon for Send/Receive, Communication Settings, Out of Office (AutoreplyP, and also an option to filter by Status with the dropdown.
Click the New icon to create a new SMS. Fill in all details and click the Send icon in the top left of the window.