Hi Vicky,

VIC-2FXS / VIC-4FXS/DID are 1st generation Analog Voice Interfaces. They are not supported on the new generation routers such as the ISRs. Whereas VIC3-2FXS/DID / VIC3-4FXS/DID are next generation and are supported on the ISR platforms. They are not backwards compatible. Kindly refer to the following data sheets which will give you more info:

Cisco 4-Port High-Density FXS/DID Analog Voice Interface

http://www.cisco.com/en/US/products/hw/routers/ps221/products_data_sheet09186a008010b355.html

Voice/Fax Network Modules for the Cisco 2600/3600/3700 Routers

http://www.cisco.com/en/US/products/hw/routers/ps259/products_data_sheet09186a0080091ba1.html

High-Density Digital Voice/Fax Network Modules for Cisco 2800/2900/3800/3900 Series

http://www.cisco.com/en/US/prod/collateral/routers/ps259/product_data_sheet09186a00800921e7_ps274_Products_Data_Sheet.html

HTH. Please rate this post if it answers your question.

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Harmit.