First check if the SMS feature is activated on your account. If it is activated, you will see "Send SMS" button next to Refresh and Send Doc buttons at top right of any Logics case.
If the SMS feature is not activated on your account, open a help desk ticket and request for the activation. Then let us know how many SMS numbers you need and in what area code(s).
Each SMS number can be assigned to one user only. To assign a SMS number to a user, go to User Management > Users > Edit a user and enter the SMS number in SMS# filed in the user profile. Click OK to save.