The low-density voice/fax network modules slide into Cisco 2600 and 3600 network module slots and contain either one or two voice interface card (VIC) slots. The VICs are daughter cards that slide into the voice/fax network modules and provide 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 VICs 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's modular branch office router products.