First enable the mobile carriers you want to list under:
Guest | Administration » SMS Services » Mobile Carriers
In your guest registration form, enable (or add) the "visitor_carrier" field and make sure it is visible.
Then enable (or add) the "auto_send_sms" field and make the sure the initial value is set to 1.
Under configuration » SMS Receipt, match the fields:
Phone Number Field: visitor_phone
Auto-Send Field: auto_send_sms
If you want to customize the SMS receipt text, modify the "SMS Receipt" template under Configuration » Print Templates.