NOTE: Only accounts with verified Mobile/SMS numbers will receive texts.
Click Org or Team Tools > Accounts in the side menu.
Click Customize Filters.
Click the Has Valid SMS dropdown and select Yes.
Select the account(s) to text.
If you do not see the desired account(s), they do not have a verified number.
Click Communicate > Email/SMS/Push Custom Message.
Check the SMS box and click Create Message.
Enter the message.
When finished, click Send Message.