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bulletWhat you need to know about Centrex Service
bulletTechnology Basics & Architecture
bulletCompatible Centrex Telephones


Centrex Features


Advantages & Disadvantages





Centrex is a service provided by your local phone company, sometimes under various service marks; for example in  New England, available is  Centrex service called CustoPak, which is a service mark of Verizon. Regardless of the name, there are several things about Centrex you should understand before buying phones to use with Centrex services.

Centrex is a service originally intended to provide multi-location businesses with the ability to tie their locations together as though (from a telephone standpoint) they were in one location. Universities with large campuses (and multiple campuses), chain stores, and other multi-location organizations have benefited from Centrex.

The big benefit is that phones in the same Centrex group can put callers on hold, transfer calls to other phones in the organization, hold conference calls across multiple locations, even page from one location to another (with appropriate paging hardware).

Accessing these Centrex features usually involves "flashing" the line and dialing Centrex access codes and/or the extension number of a destination phone (usually the last 3 or 4 digits of their telephone number) "Flashing" simply means depressing the hook switch briefly, which signals the Centrex central office that you want to put the caller on hold and access Centrex features. To make line flashing easier, any phone used with Centrex should have a FLASH KEY.

In addition, if you have regular destinations that you send a lot of calls to, you will probably want some one-touch memory buttons to store those destination extensions numbers. And, if you use any Centrex feature access codes, you may want to store these in one-touch memory buttons as well.

Last, to make call transfers easier, you may wish to store frequently transferred-to destinations with a flash, a pause, and the number, all under one memory button; to accomplish this, your phone must have a programmable flash and pause capability.



A conventional Centrex system is a physical and logical partition of a telco Central Office, which is a specialized digital computer system that incorporates a switching matrix for connecting voice calls on a circuit-switched basis.  Circuit-switched calls are temporary, continuous and exclusive in nature.

Once the call is set up through the switching matrix, the bandwidth supporting it is dedicated to that call, and that call only, for its duration. This approach is quite different from that of packet switching, which we'll revisit again in the next column on IP Centrex.

COs are very large circuit switches, commonly comprising 10,000 or more ports. They are designed to support a large number of simultaneous voice grade calls, not only between subscribers within their geographic serving, or Carrier Service Area (CSA), but also to and from the outside world of the PSTN (Public Switched Telephone Network).

As illustrated in Figure 1, the primary physical and logical components of a CO include common control, switching matrix, trunk interfaces and line interfaces.

Common Control is a set of stored program logic that controls the system and its components. Common control consists of multiple microprocessors that operate under a single stored program, which is so tightly secured that it is characterized as bulletproof. As is the case in any computer system, the control processor comprises memory, input/output (I/O) equipment, bulk memory equipment and software.

Switching Matrix design in contemporary circuit-switched COs is based on TDM (Time Division Multiplexing).

Through the use of PCM (Pulse Code Modulation), or perhaps a variation thereof, each analog voice signal is sampled and encoded into a digital format, unless that process has already been performed by the end user's CPE (terminal equipment, Key Telephone System (KTS) or PBX). Multiple digitized voice streams then are multiplexed and sent sequentially over a shared electrical bus, i.e., common physical path.

Trunk Interfaces serve to interface the CO to trunks. In the carrier world, the term trunk refers to a circuit that interconnects carrier-level switches in what is known as the trunk side of the network. So, trunks interconnect COs through intermediate tandem switches; trunks also interconnect tandems.

Trunk interfaces are in the physical form of printed circuit boards (PCBs) that fit into slots, of which there are many on a shelf, of which there are many in a chassis, of which there may be many in a large CO configuration. 

Line Interfaces serve to interface the CO to lines. In the carrier world, the term line refers to a circuit that connects a CO switch to the line side of the network, which is the end user side of the network. So, lines connect COs to single-line and multi line terminal equipment, to Key Telephone Systems (KTSs) and to PBXs.

Line interfaces are in the physical form of printed circuit boards (PCBs) that fit into slots, of which there are many on a shelf, of which there are many in a chassis, of which there may be many in a large CO configuration. 



There are several possible CO Centrex configuration scenarios, as illustrated in Figure 2. In a typical application, CO Centrex is provisioned on the basis of one voice grade twisted-pair local loop per terminal device. While this approach is very resource intensive, it generally is more cost effective than placing a remote line shelf, multi plexer or concentrator on the premises and connecting it back to the CO via a high-speed circuit such as a T1 or T3.

This latter scenario is a much more efficient use of local loop resources, as a single high-speed circuit can support a great many voice grade calls. A T1, for example, can support 24 simultaneous calls, and a T3 can support 672. In this regard, a remote line shelf is much like placing a PBX on the premises, but with much or all of the common control logic and, therefore, the call processing responsibilities remaining in the CO.


Centrex feature content is a subset of that found in the PBX domain. A PBX, of course, is an end user system. As such, it is designed around the feature requirements of medium and large end user organizations, and those feature requirements can be quite substantial and highly specialized.

A Central Office, on the other hand, is an extremely robust system designed primarily to serve the basic voice communications requirements of large numbers of end users, with emphasis on residential users, rather than businesses. All features place demands on system memory and processing power, with some features being more demanding than others, of course.

As a result, Centrex service providers generally must limit the available features to those which are less resource intensive, but which have broad market appeal and which, therefore, offer considerable revenue and profit potential without compromising the ability of the system to perform its fundamental responsibilities.

Contemporary Centrex systems offer dozens of features, including:

Automatic Route Selection (ARS), also known as Least Cost Routing (LCR)
Call Pick-Up Groups
Call Forwarding
Conference Calling
Call Detail Recording (CDR)
Voice Mail

CLASS (Customer Local Access Signaling Services) features include:

Name Delivery
Calling Number Delivery
Calling Number Blocking
Continuous Redial/Repeat Dial/Automatic Redial
Distinctive Ringing
Call Trace
Selective Call Screening

In select areas, more advanced features may be available. For example, ACD (Automatic Call Distribution) may be available through CO Centrex interfaces to specialized ACD platforms.


The advantages of conventional Centrex systems can be numerous when compared to conventional PBX systems. These advantages stem from the fact that the telco owns the system, and assumes all the responsibilities that go along with it.

A short list of the burdens of ownership includes the following:

Cost of acquisition
Cost of ongoing operation, including maintenance
Cost of upgrades and enhancements
Cost of insurance
Cost of power systems, both primary and backup
Cost of fire suppression systems
Cost of floor space
Cost of physical and logical security
Cost of replacement
Cost of redundancy

The disadvantages to the end user organization also can be considerable in comparison to conventional PBX systems. Negatives include the following:

Cost of monthly service charges
Lack of feature content
Loss of control

Centrex is pretty boring stuff, as systems go. The challenges of ownership belong to the telco, and so does all the excitement that goes along with them. Centrex is highly reliable, as it is a partition of a CO, and COs are among the most highly reliable systems in the world.

COs are operated and maintained by telcos, which are among the most highly reliable companies in the world. Sometimes boring is good. (I worked for telcos for 15 years. I left before I became bored to death. Boring was not good for me ... as an employee, at least.) While Centrex systems don't offer all the feature content that many companies would like, what they do offer works and works and works. Again, Centrex systems are highly reliable.


Centrex systems suit certain applications perfectly. Organizations that operate under tight capital budget constraints find Centrex very attractive, with examples being public sector organizations such as federal, state, county and municipal governments; hospitals; public schools and institutions of higher learning.

Highly seasonal businesses find Centrex attractive, since Centrex lines can be connected during the busy time of the year and disconnected when business is slow, and with the capital expenses of supporting communications during the busy season being those of the telco.

Multi site organizations, particularly those confined to a relatively small geographic area, find Centrex highly appealing. A healthcare complex, for example, can include the main hospital, an administration building and a number of clinics, all of which commonly are clustered in an area of a few city blocks.

All of these telephones in these buildings can be served by a single CO Centrex system, with a single user interface and at very reasonable cost. This compares very favorably with the alternative of premises-based KTS and PBX systems, which commonly would interconnected with costly dedicated circuits in a full mesh or partial mesh network configuration. Such an alternative can be very expensive and highly complex.




Aastra Meridian M5008 (1-Line appearance)

The M5008 is perfect for a reception area, construction floor, secondary department phone, or other low-volume calling applications where a display is unimportant but reliability and ease of use are priorities.  "Digital" centrex service required



Aastra Meridian M5208 (1-Line appearance)

The M5208 is a good match for arrangements or other applications where occasional call coverage and exceptional reliability are important.  "Digital" centrex service required



Aastra Meridian M5216 (1-Line appearance) 

The M5216 is recommended when you need a central answering position or ACD support. It offers unique functionality, making it the perfect solution for call centers or other high-volume calling applications. "Digital" centrex service required




Aastra Meridian M5312 (1-Line appearance)

The  M5312 Automatic Call Distribution (ACD) telephone is ideal in situations where agents must handle a large number of calls quickly.  "Digital" centrex service required





Aastra Meridian M5316 (1-Line appearance) 

The M5316 is the ideal set for busy professionals who conduct much of their business by phone. This set offers important time-saving and productivity features.  "Digital" centrex service required



Aastra Meridian M9417CW (2-Line appearance)

Take a two-line phone, add all the helpful features you can imagine - including Call Waiting Display - and you've got the best tool for professional, efficient communication.  Works on "Analog" centrex service



Aastra Meridian 9316CW (1-Line appearance)

With the M9316 on your desk, there's no need to flip through a phone book--you've got your own personalized directory. Simply enter up to 50 names and numbers, and the most important callers are always right at your fingertips. Use the scroll keys or the keypad to find the number you're looking for, then press the dial key. It's that simple.  Works on "Analog" centrex service



Aastra Meridian M9216 (1-Line appearance) 

The Nortel Meridian 9216 phone features Name and Number Call Display*, and a call log with name, number, date, and time Callers List*. This phone is compatible with Centrex, PBX, and standard telephone service with Link, Hold, and Release keys.  Works on "Analog" centrex service


Aastra Meridian M6320 

The TAPI compliant M6320 is the essential advanced business set.

Request price & AVAILABILITY


ITT Cortelco 2200

1-Line with Caller ID/Call Waiting, Speakerphone



ITT Cortelco 2210


1-Line with Caller ID/Call Waiting, Speakerphone

Request price & AVAILABILITY


ITT Cortelco 2220  

2-Line with Caller ID/Call Waiting, Speakerphone, Conference

Request price & AVAILABILITY


TMC Epic ET4300 (4-Line) 

Axxess up to 4 telephone lines
Caller ID & Call Waiting capability on all 4 lines
Visual Message Waiting Indicator
Headset Jack
3-Way Conference Calling
Data/Fax Jack
Backlit Pivoting LCD Display
Up to 16 stations
Call Transfer
Shared Directory Dialing




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