"Similar to the Battle.net Authenticator and Mobile Authenticator application, the Battle.net Dial-in Authenticator is an optional tool that provides an additional layer of security against unauthorized account access. The Battle.net Dial-in Authenticator is not a physical token or application run on a mobile device, however. Instead, it is a free opt-in service that will actively monitor an account and request additional authorization from the user when a potentially unauthorized login attempt occurs."
The service asks you to nominate a phone and a PIN. When your account is accessed from a different IP address it will ask you to authenticate by dialling a US toll-free number from your nominated phone and entering your PIN and a single-use security code. The service is optional and best of all, it is free.
This is a good addition to the security arsenal, especially for those users that move around a lot and don't have a hardware authenticator. Remember that good security is made up of several layers of protection, and this offers yet another layer.
More information can be found at the official Battle.net Dial-in Authenticator FAQ.