Usually during the two-factor authentication process a one-time password is used to verify the user's identity. This secures authentication by asking for multiple criteria to be met such as something you know and something you have. Something you know being your traditional username and password and something you have being your OTP or one-time password. However during transmission of this one-time password a hacker could still intercept the data to gain access if the OTP is not sent to an out-of-band network.
One-time passwords come in many forms from something as simple as a sheet of codes to the more advanced propriety key generating tokens. Many times for information that is not an extremely high security risk the OTP will be sent via email to the user for identification. This is not an out-of-band solution because the email can be received on the same network as the login panel.
The problem with sending the second factor in the authentication process to a solution that is not out-of-band is that easy to use and readily available software makes it easy to intercept information including the user's one-time password. With an out-of-band solution the user would need to receive their OTP on a separate network than their login panel. One way is through proprietary tokens that generate dynamic one-time passwords. However tokens be pricey and can create havoc when lost or misplaced.
Another less expensive and more reliable device would be the user's mobile phone. Since we are a society who must be connected to our mobile phones constantly a user will not forget their device and the chances of the device being broken are much lower. Also the device's network is completely out-of-band from any login panel.
Securing authentication by sending the one-time password through an out-of-band network protects the user from malicious software as well as misplacement of their device. This makes it very hard for a novice hacker to gain access to confidential information or networks and ensures the user will receive their OTP when they need it.
The only way to become more secure once you already utilize an out-of-band OTP for two-factor authentication is if it is a zero footprint solution. Zero footprint authentications allow the one-time password to be sent without leaving any trace of the authentication or password behind on the device. Ultimately securing the authentication process completely from internet or network based attacks.