SMA100: Why is SMS grayed out in OTP configuration?

When configuring One Time Password (OTP) on an SMA 100 by default the SMS option is grayed out. 

To select SMS as the OTP method the configuration must support SMS.  If the SMS and email configurations are not done, SMS is not an option. 

To support SMS OTP you must complete the underlying configuration.

- Email must be configured under Log > Settings > Event Logging and Alerts.

- An SMS template must be configured under Services > SMS Templates. 

With email and the SMS Template configured SMS becomes a valid option under the OTP configuration. 

In the following screen shot, SMS is an option now that an SMS template exists: