http://cisco.com/en/US/products/hw/routers/ps259/products_data_sheet09186a0080091b9c.html
These 2-port Multiflex VWICs simplify branch office connectivity by enabling a Cisco 2600, 2800, 3600, 3700 or 3800 to consolidate the functions of a router, a fully managed Drop and Insert Multiplexer, and a fully managed DSU/CSU into a single box. Typically a Drop and Insert multiplexer is used for channelized (i.e. Time Division Multiplexing [TDM]) integration of voice and data onto a single T1/fractional T1 or E1/fractional E1 connection to the CO. Sharing a line can significantly reduce costs over that of two separate physical lines to the CO. While the normal use is for voice and data sharing of a T1 or E1 service, the Drop and Insert capability can also be used for video and data, or data and data sharing of the service (e.g. two routers).
Moreover, the Enhanced Availability Drop and Insert (EADI) feature enhances system availability by allowing the Cisco IOS software to be reloaded while maintaining time division multiplexing (TDM) switching.