To support analog Foreign Exchange Station (FXS) or Foreign Exchange Office (FXO) voice ports on Cisco 175x Series Access Routers, you need the Voice Interface Card (VIC)-2FXS or VIC-2FXO (either FXS or FXO VIC, or both).
The Digital Signal Processor (DSP) card is called a Packet Voice DSP Module (PVDM), which is installed on the motherboard of the Cisco 175x Series.
The Cisco 1750 platform motherboard has a slot for PVDM modules that provide compression of voice calls so there is no need for an extra Network Module (NM)-2V to house the VIC card.
Make sure your Cisco IOS Software image has the IOS Plus feature set. This is denoted as the "-sv3y" on the image name. If this is not present, the voice hardware is recognized by the platform.
For more information on the FXS or FXO VIC and Cisco IOS Software requirements, refer to these documents:
Note: Calls from the Cisco 175x platform are always run in high complexity mode for the C549 technology type DSP. Therefore, one DSP can handle two calls at most, whereas on other platforms the same DSP can handle either four calls (medium complexity) or two calls (high complexity).
Below are the different combinations of PVDMs for Cisco 1750 and Cisco 1751 platforms.
PVDM-4. Supported on both the Cisco 1750 and Cisco 1751. Contains one DSP and processes two calls (1 FXO/FXS VIC card).
PVDM-8. Supported on both the Cisco 1750 and Cisco 1751. Contains two DSPs and processes four calls (2 FXO/FXS VIC cards).
PVDM-12. Supported on both the Cisco 1750 and Cisco 1751. Contains three DSPs and processes 12 calls (3 FXO/FXS VIC cards).
PVDM-16. Supported only on the Cisco 1751. Contains four DSPs and processes 16 calls (4 FXO/FXS VIC cards).
PVDM-20. Supported only on the Cisco 1751 Digital Signal Processor. Contains five DSPs and processes 20 calls (5 FXO/FXS VIC cards).