Important Note: If you are using the Classic version, please click here to see the instruction. Below explained instruction is applicable only for the Lightning theme.
By default, there will be List view Bulk SMS button for Contact, Account, Lead, Campaign, and Case. (If you are unable to see, access the same from search layout > List View)
if you are using Lighting, you have to create a button by doing below instructions.
Step 1) Create a Page
Navigation : Setup > Custom code > Visualforce pages > Search for “VtBulkButtonMaker” page > Click on “Preview” button
Select the object where you want to create button and click on “Create Bulk Button Page” button
Step 2: Create a Button
Navigate to below :
Lightning theme Instructions: Setup > Object Manager > Object > Buttons > Click on New Button or Link > Select Display Type asList Button and Content Source asVisualforce Page
Now Select the page which you have created in Step 1 and save it.
Step 3: Assign the button to Layout
Navigation : Setup > Object > Search Layout > Edit “List View” > Drag the button to “Selected Buttons” section and save it.
Bulk SMS feature is now ready for use.