When signing a document, signers must authenticate themselves to verify their identity. Authentication is possible through those different options:

No security code: Only for non-critical signature requests or for in-person signing. Your recipient will not need to authenticate him- or herself to sign documents, only to click on the "Sign" button of your invitation. This applies to most signature requests, for non-critical documents.
This is the authentication mode by default on your application.

Security code by SMS: Your recipient will receive a security code by SMS, a "one-time password" needed to sign documents. This code will be sent to the phone number entered when creating your contact. This applies to documents requiring a higher level of security.
Security code by e-mail: Your recipient will receive a security code by e-mail, a "one-time password" needed to sign documents. This code will be sent to the e-mail address entered when creating your contact. This applies to documents requiring a higher level of security or when you don’t have signers’ phone numbers.

Phone Authentication: If you enter a landline number for your recipient, he will then receive a phone call from a voicemail with the security code. For this option, we recommend that you inform your recipient to stay near his landline when signing.

If your recipients are encountering difficulty during the authentication stage, please refer to the help articles below.

Sign a signature request

How do I sign a document?