This is the card we suggest as a much cheaper serial port option for the 4xxx series routers. This is only capable the the 4xx series routers (You'll see the 4321 and 4331 on our site).
The Cisco Fourth-Generation T1/E1 Multiflex Trunk Voice and WAN Network Interface Modules (NIMs) support data and voice applications on the Cisco 4xxx integrated services routers. These cards combine WAN-interface-card (WIC), voice-interface-card (VIC), ISDN Primary Rate Interface (PRI), dial-access integration, and channelized-data (CE1T1) functions to provide unparalleled flexibility, versatility, and investment protection through their many uses. Customers who choose to integrate data and voice in multiple steps preserve their investment in a T1/E1 WAN interface. The T1/E1 modules contain an onboard Cisco Packet Voice Digital Signal Processor Module 4 (PVDM4) slot, which requires a fourth-generation PVDM4 module. A PVDM4 on the T1/E1 module is necessary for the voice features. The PVDM4 also provides for echo cancellation of up to 128-ms echo-tail length for demanding network conditions.