This feature is only available for the Business plan.
This feature can then be used as follows: 

1. Upload your document.

2. While adding the signers, check the box below each signer "Authenticate Signer".

3. Select the SMS verification.

4. Enter the phone number of the Signer.

5. Now when the signer receives the document, they must enter an OTP sent on that phone number in order for them to be able to sign.