Used Cisco 3600 & 3700 Expansion Module - 16-Channel DSP Module for NM-HDA 

About the Cisco 3600 Series: "Modular high-density router for dial-access or router-to-router traffic. Supports ISDN, serial, channelized T1, digital modems, and ATM links. Also supports voice/fax and Frame Relay" (Cisco A Beginner's Guide Third Edition by Velte & Velte).

Part # DSP-HDA-16

Product Description: 16-Channel DSP Module for NM-HDA. The 16-channel DSP Module provides enterprises, managed service providers and service providers the ability to directly connect the PSTN and legacy telephony equipment to Cisco 2600, 2800, 3600, 3700, and 3800 Series access routers. This provides immediate multi-service advantages, such as telephony toll bypass, new packet telephony applications, and full gateway integration within a Cisco AVVID (Architecture for Voice Video and Integrated Data) IP telephony architecture.

Directory of Cisco 3700 Expansion Modules

Directory of Cisco 3600 Expansion Modules