The Cisco 4-port Network Interface Module - FXO (Universal) (Model: NIM-4FXO) is a 4-port analog voice module designed for Cisco 4000 Series Integrated Services Routers (ISR 4000). This module acts as a bridge between an IP network (VoIP) and traditional telephone lines (PSTN/PBX). Key Functions and Purpose – FXO (Foreign Exchange Office) Ports: The module features 4 analog line ports. Through these ports, the router connects directly to a public telephone network (PSTN) or an organization's internal private branch exchange (PBX) system. Universal Support: It is called "Universal" because the module can be easily adapted—at the software level—to the telecommunications standards and signaling types (Loop Start / Ground Start) of various countries. VoIP Gateway Role: It converts analog voice signals into digital IP packets (SIP, H.323, or SCCP), enabling operation within a Cisco Unified Communications Manager (CUCM) or Cisco Unified Survivable Remote Site Telephony (SRST) environment. Technical Specifications Feature Detail Model Code (SKU) NIM-4FXO Number of Ports 4 x FXO (RJ-11 / RJ-45 physical connection) Compatible Devices Cisco ISR 4000 series routers (ISR 4321, 4331, 4351, 4431, etc.) PVDM / DSP Includes an onboard DSP (Digital Signal Processor) chip Supported Signaling Loop Start, Ground Start Additional Features Caller ID, Battery Reversal Detection, Cable Detection OIR (Hot-Swap) Supports Online Insertion and Removal (inserting or removing the module without powering down the router) Use Case: Backup Voice Line: Allows switching to public telephone lines (PSTN) for emergency calls in the event that the primary IP telephony service (SIP Trunk) fails. Legacy PBX Integration: To connect an existing analog PBX system to the new Cisco IP telephony infrastructure. Fax and Analog Device Connectivity: To route faxes or specialized access systems that operate over analog lines.