Hi Alex
I've deployed this once before. First off the SMS verification requires redirection to NAC's captive portal through either PBR or DNS proxy. You then need to enable SMS verification and edit the custom fields with "Mobile Number" as mandatory so user is forced to provide mobile number. You can choose from a combination of passwords ie. Numerical, alpha-numerical, alphabetical and so on. As you've mentioned you'd need to integrate with an SMS gateway supported by NAC/NMS.
Connection process will be as follows:
User connects to SSID>>>User gets redirected to captive portal>>> User registers with details including mobile number>>>NAC/NMS sends user a verfication code via SMS>>>New portal shows in browser where verfication details are required>>>User inputs verification code>>>Network access is granted.
The network access will obsiously be determined by the returned NAC profile. Guest access was used in my scenario.
Hope this helps
Regards,
Francois