We already have this functionality built in.
Add a new SMS Gateway for SMS over SMTP:
Under Mobile Carriers, enable the carriers that you want to support (CPG->Administration->SMS Services->Mobile Carriers).
Edit you Guest Self-Registration Form:
Click on visitor_phone, and then click Insert After. Choose visitor_carrier from the drop-down list. Under Form Validation Properties, check the box for Field value must be supplied:
Click Save.
Go to the Advanced editor at the bottom of modifying your guest self registration page (so image above). Scroll down to SMS Delivery options. Choose Always auto-send guest receipts by SMS, Service Provider: SMS over SMTP. Save.
Over in ClearPass Policy Manager, go to Administration->External Servers->Messaging Setup. Add in your SMTP settings here.
That should be it. Now, as long as your guest choose the appropriate carrier when registering, they should receive their credentials via SMS.
One other thing I would recommend is that you hide the password from the online receipt and disable auto-login (just add a link back to the login page in your receipt page footer, disable download receipt, and disable the login button on the Receipt Page form). That way you make sure they are giving you the correct mobile information before they can get online.