Contacts object will be used to Send SMS in below way:

Creating new Contact record:

Step 1:

  1. Click on Contacts Tab. 

  2. Click on New button. 

  3. Fill all the fields with desired Mobile/Phone. 

  4. Click on Save button. User will be navigated to contact's Details page after save.

User can either Edit, Delete the newly added Contact using respective buttons.

Step 2: 

Once added, click the Contact from where message is to be sent.

Step 3:

Open contact detail page, You could see below component. It displayed message i.e. "Phone Number Does Not Exist On Mapped Field. " since contact number doesn't have mobile number.

Step 4:

Now enter the mobile number, save. it would display message,"There are no messages yet, please start conversation by sending one! ".

Step 5:

Enter message and click Send button, message will be sent and below success message will be displayed. 

Confirmation message like above will be displayed once the message is sent.

This is how Contacts tab help sending the SMS.

Alternatively, user can send the message using the side component as provided on the Leads/Contacts page.

This is explained on the other help link.