Cisco - Serial Network Modules
Cisco - Serial Interface Cards
Cisco - Cisco Modular Access Router Cable Specifications
Cisco - Cisco Voice Modules and Interface Cards
Cisco - Understanding Foreign Exchange Station (FXS) Voice Interface Cards
Cisco - Voice Interface Cards
Cisco - Connecting Cisco 4-Port FXS/DID Voice Interface Cards
Cisco - Cisco IP Communications Voice/Fax Network Module
Cisco - Understanding NM-1V and NM-2V Voice Network Modules
Cisco - Understanding High Density Voice Network Modules
Cisco - Understand the NM-HDV2 IP Communications High-Density Digital Voice or Fax Network Module
Cisco - Understanding High-Density Analog Voice/Fax Network Modules (NM-HDA)
Cisco - Understanding E&M Voice Interface Cards
Cisco - Cisco Network Modules - Install and Upgrade Guides -Voice Network Modules
Cisco - High-Density Packet Voice Digital Signal Processor Module for Cisco Unified Communications Solutions for PVDM2.
Cisco - High-Density Packet Voice Digital Signal Processor Module for Cisco Unified Communications Data Sheet for PVDM3.

Cisco - Installing, Replacing, and Upgrading Components in Cisco Modular Access Routers and Integrated Services Routers

Cisco - Master and Slave Relationships on a T1 Back-to-Back Connection
Cisco - Troubleshooting the DSP on NM-HDV for Cisco 2600/3600/VG200 Series Routers
Cisco - DSP on NM-HDV2 Functionality Verification for 2600XM/2691/2800/3700/3800 Platforms

Cisco - Overview of Cisco Network Modules and Service Modules for Cisco Access Routers
Cisco - One- and 2-Port Serial and Asynchronous High-Speed WAN Interface Cards for Cisco 1800, 1900, 2800, 2900, 3800, and 3900 Series Integrated Services Routers
Cisco - Cisco One and Two Port T1/E1 Multiflex Voice/WAN Interface Card
Cisco - Cisco Digital 1-port and 2-port T1 Multi-Flex Voice WICs
Cisco - WAN Interface Card (WIC) / Platform Hardware Compatibility Matrix for 1600, 1700, 2600, 3600 and 3700 Series Routers
Cisco - High-Density Digital Voice/Fax Network Modules for Cisco 2800/2900/3800/3900 Series

Cisco - AIM-ATM Documentation Roadmap
Cisco - AIM-ATM, AIM-VOICE-30, and AIM-ATM-VOICE-30 for the Cisco 2600 Series and Cisco 3660
Cisco - Understand Advanced Integration Module (AIM) Usage in ATM Technologies
Cisco Blog - What is the difference? AIM-ATM AIM-ATM-VOICE-30 AIM-ATM-VOICE-30
Cisco - End-of-Sale and End-of-Life Announcement for the Voice/Fax and ATM AIM Modules for Cisco Multiservice Access Routers
Cisco - Installing Advanced Integration Modules in Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series Routers

Cisco - CEoIP Network Modules
Cisco - End-of-Sale and End-of-Life Announcement for the Cisco 4-Port T1/E1 RJ-48 Circuit Emulation over IP and Serial Interface Network Modules
Cisco Community - Circuit Emulation over IP (NM-CEM-4TE1)

Cisco - CISCO 1800 Series Router Hardware View
Cisco - CISCO 2600 Series Router Hardware View
Cisco - CISCO 3600 Series Router Hardware View
Cisco - CISCO 3700 Series Router Hardware View
Cisco - CISCO 3800 Series Router Hardware View

DMS-100.net - Cisco IOS configuration notes with many T1/E1, modem, voice configuration examples.

Nesevo Wiki with many Cisco (router) password reset information and other info.

Cisco - Cisco Integrated Services Routers Generation 2 Ordering Guide
Cisco - Cisco 2900 Series Integrated Services Routers Data Sheet

My Network Lab - Cisco 899G-LTE Dual SIM Configuration

Cisco - E1 R2 Signaling Theory
Cisco - E1 R2 Signaling Configuration and Troubleshooting
Cisco - E1 R2 Customization with cas-custom Command
Cisco - Configuring and Troubleshooting E1 R2 Signaling for Data Calls
Cisco - E1 R2 Signaling for the Cisco AS5800

Taken from; YouTube - Cisco ISR G2: New Modules! (Excerpt from TechWiseTV53) - 0:29 on 2009-11-05, by ExplaiNerds. Also shown in Slidesshare - Cisco isr g2.

NetworkLessons.com - IOS Licensing: 1900, 2900, and 3900 series routers use a universal IOS image that provide only the IP Base feature and require installation of a license key for features beyond IP Base. Routers before the mentioned series used an IOS image per features set, and to get a certain feature set you only needed the correct image to get that feature set.
The Cisco Learning Network - Understating Cisco IOS v15 Licenses (Yes, “understating”. There are more spelling errors in the text of the article.) Mentions that IOS 15 is (only?) provided as a universal image. That would make IOS 15 require feature set licenses too?
Cisco Community - 2811 router IOS 15 licensing ? suggests that IOS 15 does not require license activation on a Cisco 2811 router, but it does on G2 routers like 2900, and 3900.
Cisco Community - Home Cisco router WITHOUT licensing

YouTube - LabMinutes# RS0001 - Cisco Router Back-to-Back T1 Configuration on 2012-08-17 by Lab Minutes. Shows an example of connecting 2 routers back-to-back with ISDN PRI and usage of “isdn protocol-emulate network” on se0/0/0:23 from 19:31 onward.