The VIC is a daughter card that slides into the voice/fax network modules and provides the interface to the telephony equipment and the PSTN. Just as the Cisco WAN interface cards can be swapped with other WAN interface cards, the Cisco VIC can be deployed interchangeably with other VICs in the voice/fax network modules. This built-in flexibility and investment protection are key reasons for the worldwide popularity of Cisco modular branch office router products.