SMS can be used to as an authentication (2FA) method in two-factor authentication. You can use a built-in 2FA account to handle SMS authentication, but if you have an SMS account configured for messaging, you can also use that account to handle authentication.

1. To enable an SMS account for 2FA, when configuring your SMS service, select Two factor authentication under Usage.
   
2. Go to Setup > Access > Two Factor Authentication.
3. In the Two Factor Authentication screen, select Text message (SMS) under Authentication Method and confirm the message content. 
   
4. In the SMS Account drop-down, select the SMS account you configured above.
5. Choose whether to use an Alphanumeric Sender ID for the messages. An Alphanumeric Sender ID is used instead of a phone number to serve as the sender for the authentication codes, and is required in some countries. If you select the Use Alphanumeric Sender ID checkbox:
   1. Enter the ID you want to use. Note that some countries require you to pre-register your Alphanumeric Sender ID before you begin using it. Make sure to check the requirements for any countries you operate in.
   2. Select the countries in the list where you want to use the Alphanumeric Sender ID. Hold Ctrl to select more than one. If you accidentally select a country that doesn't support Alphanumeric Sender ID, the system automatically uses the phone number instead.
6. Click Finish.

Now, when someone logs in, they receive an SMS message with a security code.