Surveys can be protected by a PIN based on the mobile number. Using the API the PIN can be set independent of the mobile number.
For example, when a person clicks a PIN protected survey link they receive via SMS the person will be requested to enter the last four digits of their mobile number. The content of the survey will only be displayed when the correct PIN is entered.
If the person enters the PIN incorrectly three times the page will 'expire'.
Here's how to enable it via the dashboard.
From the campaign launcher click the 'Security' button for the campaign.
Click the toggle to 'On' and you'll see the following options.
PIN Instructions - Simple instructions that are displayed to the user.
Number of Attempts - Limit the number of attempts the user has to enter the PIN correctly. After this number the page will expire
Generate from Phone - by default the last 4 digits of the mobile number are used as the PIN