Call Center Phone Systems

call center icon

What Is A Call Center?

Call centers special offices that are purpose-built to handle a large volume of phone calls. Call centers typically handle customer service, support, telemarketing, telesales and collections functions. The employees who staff call centers are referred to as “agents” or “customer service representatives” (frequently abbreviated as CSRs). Call centers range from very small informal operations to massive, highly optimized sites with hundreds or even thousands of agents.

Call centers use specialized telephone equipment to maximize productivity. Specialized telephony switching systems called “Automatic Call Distributors” or ACDs are used to queue and route inbound calls to agents based on a wide variety of criteria. Outbound calls are frequently generated by an automated system called a “Predictive Dialer” that monitors the status of agents and place calls on their behalf. Other common call center tools include desktop integration (frequently referred to as “screen pop”), Interactive Voice Response (IVR) applications, call recording solutions, productivity monitoring utilities, workforce planning systems and various methods of historical and near real-time reporting.

Call Distributor Icon

Automatic Call Distributor (ACD)

An ACD is a specialized phone system that routes (distributes) incoming calls to teams of agents assigned to various call queues. Queues are simply ordered list of calls to be dispatched to agents. The ACD oversees the process of placing incoming calls into the proper queue, assigning priority to those calls based on various factors (the order of their arrival, the importance of the caller, the urgency of the caller’s situation), and ultimately dispatching those calls to an available agent. The algorithm by which calls are dispatched is called the queue strategy.

Call queue icon

Queue Strategies

A simple ACD system consists of a source of calls (a pool of lines, trunks or virtual trunks), a FIFO (first-in, first-out) queue and a pool of agents who are selected using a “ring-all” strategy. In this case, when a call arrives the system rings the phones of all agents who are not already on a call. The first agent to answer the call is connected with the calling party. All the other phones stop ringing.

A more complex (and likely more useful) configuration would have the call offered to the agent who had been in the idle state longest. This “most idle” strategy is frequently used when all agents are considered equally qualified to handle a task. Other common strategies include round robin, linear hunt, least-recently-called, fewest calls and random. In some cases, the ACD can weight its selection based on the caller’s need (generally collected using an IVR application) and a list of skills associated with each agent. This is generally referred to as “skills-based routing”.


Caller Experience

While waiting in queue, callers generally hear a combination of marketing messages, queue status messages and music. Marketing messages are simply audio recordings that are piped into the queue on a periodic basis. Status messages provide the caller with specific information about their status — the number of callers ahead of them in the queue, the estimated wait time and sometimes alternatives to waiting in queue. Some more advanced call queueing systems support virtual queueing. A virtual queueing system allows callers to provide a callback number, then disconnect. Their position in the queue is preserved and when an agent becomes available the system places an outbound call to the caller.

Call dialers icon


Outbound call centers frequently use a dialer application to connect agents with targets. Dialers can be simple desktop applications that implement a basic “click-to-call” function or much more advanced systems

Desktop Dialer Icon

Desktop Dialer

Desktop dialer applications are generally integrated with Customer Relationship Management (CRM) software or other business applications. When an agent needs to make a call, the click a button or link in the dialer application rather than manually entering the digits on the keypad of their phone. The dialer application then places the call on their behalf by sending a command to the phone (known as “first person” integration) or to the phone system (known as “third person” integration). Automating the dialing process reduces errors and increases productivity.

Call Power Icon

Power Dialer

Large-scale cold-calling operations commonly associated with tele-sales, collections, and political campaigns, require more aggressive dialer systems. Some of these are simply “power dialers” which essentially place one outbound call for each available agent. When an agent hangs up, the system automatically dials another call and connects it to the agent’s phone. This pattern continues as long as the agent remains logged into the system. Power dialers allow an outbound call center to place far more calls than could be accomplished manually. However, there is still some agent time wasted handling calls that do not connect, reach answering machines or otherwise fail to reach the target.

Call Predictive

Predictive Dialer

Predictive dialers are essentially smarter power dialers. They carefully monitor the average handle time for each agent and attempt to predict when an agent will become available. Rather than placing calls on a one-agent-to-one-call basis, they place more calls than there are agents available. When a call is answered the system uses various methods to determine if the answering party is a human or an answering machine. Answering machines are either dropped or fed a pre-recorded message while live answers are handed off to agents.

Robo Caller Icon

Robo Dialer

Sometimes dialer systems are completely automated. These place calls then play messages to the answering party or machine. These systems are commonly used for notification purposes (i.e. reminder of a doctor’s appointment, notice that school has been canceled) as well as marketing and political messaging. Some automatic dialer systems support additional features like surveys or transfers to live agents.

Dialer Considerations

Dialer Considerations

In most jurisdictions, there are many laws governing the use of dialers. Before deploying a dialer be certain to review these to avoid significant penalties or even criminal charges.

Adjunct Technologies

While ACDs and dialer systems are quite powerful on their own, interconnecting them with a number of related technologies has the potential to increase the efficiency and in some cases improve customer experience.

Automated Attendant

Automated Attendant

Automated attendant systems have long been paired with ACDs, allowing callers to route themselves into the appropriate call queue. Automated attendants are simply menu systems that prompt callers to indicate their preference using the keys on their phone or, in some cases, by speaking keywords. Callers are generally willing to accept up to two levels of menu before reaching a live agent. More than two levels tend to annoy most callers and can result in an increase in abandoned calls.

Interactive Voice Response Icon

Interactive Voice Response

Interactive Voice Response or IVR applications are another technology frequently integrated with ACDs. IVR systems prompt callers for data items — things like account numbers, prescription refill codes or package tracking numbers — and use those values to look up caller-specific information from remote data sources. In some cases, the IVR application can handle an entire transaction without human intervention. In other cases, the IVR feeds data to the ACD to help it route the call appropriately.

CTI/ Screen Pop Icon

CTI / Screen Pop

To increase agent productivity, desktop business applications are frequently integrated with the ACD in such a way that data related to the caller is automatically displayed when a call is delivered. This is commonly referred to as either “Computer-Telephony Integration” or simply as a “screen pop”. There are several ways to accomplish this. In some cases, the business applications support a CTI standard like TAPI or TSAPI. In other cases, the applications are custom and communicate directly with the ACD or a “CTI server” that acts as a proxy for the ACD.

Call Recording

Call Recording

Call centers frequently record calls either to monitor the performance of their agents or for regulatory compliance. Call recording systems handle the process of capturing the audio from all participants in the call, mixing it, storing it and producing an index that allows administrators or regulators to locate and review recordings. A properly built recording system makes it easy to pinpoint conversations using common keys including Caller ID, date, time and agent ID.

Asterisk In The Call Center

Asterisk is a powerful tool for building call center systems and solutions. With support support for call queues, IVRs, outbound dialing, recording, live monitoring and reporting, Asterisk includes virtually everything you need to create a working call center. Small and informal call centers can be built using a single Asterisk server or deployed from a turn-key IP PBX. Enterprise call centers generally make use of a cluster of Asterisk systems structured to scale as the business grows. Call centers with legacy ACD systems frequently use Asterisk as an adjunct, acting as the IVR front-end to a skills-based routing solution.

You can find a number of pre-built Asterisk-based call center solutions on the AsteriskExchange.

What can we help you find?