Telephony Signaling

Document ID: 25800

Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
Problem
Solution
Related Information

Introduction

This document explains why a telephone approved for the United Kingdom (UK) may fail to ring when connected to a Cisco Foreign eXchange Station (FSX) port and explains how to correct the failure.

The failure results from a physical interoperability issue and is independent of Cisco hardware or software. British Telecom did not implement RJ11 type connectors when it adopted a plug and socket connection methodology. RJ11 connectors allow parallel connectivity for the transmission path and the ringer circuit. The reason they weren't used was that older telephones had needed to have their ringer circuits connected in series because the ringing devices needed high current.

Outside the UK, ringer circuitry is self-contained in each phone. The UK implementation puts the capacitor, which provides the AC ring path, and the anti-tinkle feature (prevents the bell or ringer from sounding when pulse dialing is used) externally in the first socket, connected to the local loop.

Prerequisites

Requirements

There are no specific prerequisites for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

For more information on document conventions, see the Cisco Technical Tips Conventions.

Problem

In the UK certain British Approval Board for Telecommunications (BABT) telephones fail to ring when they are connected to FXS ports on Cisco voice-enabled routers and switches. Outgoing calls can be made and voice communication in both directions can be established, however with incoming calls, the telephones will not ring. These telephones functioned correctly before being connected to the FXS ports.

Solution

Because a proprietary connection system is implemented, you must use an adapter to connect the telephone to an FSX port. The adapter must be a master that contains the capacitor or the telephone will fail to ring.

The following schematic is a diagram of the master socket. Notice that there is an additional third connection wire (3) to the telephone that is used to provide the series path for the ringing. The over-voltage spark gap is used to provide a short circuit if the line is accidentally introduced to high voltages (for example, lightening). Additional telephones only need to connect across the line on connections 2 and 5.

2--------+------------------+--------------------2
          |                  |
          |                 ---
          |                 --- 1.8uF 250V
          |                  |
          V                  +--------------------3
           
            Overvoltage      |
          ^ Spark gap        \
          |                  / 470k
          |                  \
          |                  /
          |                  |
  5-------+------------------+--------------------5

Related Information

• Voice Technology Support
• Voice and Unified Communications Product Support
• Recommended Reading: Troubleshooting Cisco IP Telephony
• Technical Support - Cisco Systems