To send a bulk message, you will need to add the Send Batch SMS button to your List View search layouts.
The package will require you to use the Bulk Message feature in Salesforce Classic.
This example will show you how to add the Send Batch SMS button to the Contact object.
Sending a Bulk Message
Step 4: Add batch send to Search Layouts
Select the Batch Send SMS button under Custom Buttons.
Click the arrow to add Batch Send SMS Button to the List View Search layout.
Save your search layout.
Remember that it can take a few minutes before the button shows up on your Search Layout. Try hard refreshing (CTRL+F5) if it doesn't show up.