Anatomy of VOIP call centre


Call centre

Photo by Carlos Chavez

Businesses need to make multiple contacts with their potential and existing customers everyday, not always by physical meetings. Where the contacts are done via telephone, usually these activities are centralised in call centres.

  • Inbound call centre is a call centre which receives inbound calls from existing, and occasionally potential, customers. The topics of the calls are usually questions or complaints about products or services, with occasional requests to buy
  • Outbound call centre is a call centre which makes outbound calls to potential, and occasionally existing, customers. The topic of the calls is usually offer to sell products or services

Voice Over Internet Protocol (VOIP)

Photo by Maria Li

Remote voice transmission was initially done as analogue signal across analogue telephone network since invention of telephone by Alexander Graham Bell in 1876. In 1972, an algorithm called G.711 enables the conversion of analogue to digital voice signal, so voice can be transmitted without loss of quality in transmission and core network of telecommunication companies (since then, newer algorithms have been created to do this even more efficiently). As Internet Protocol (IP) slowly becomes single protocol used within intranet and internet, International Telecommunication Union (ITU) released the H.323 standard in 1996 to govern digital voice transmission over intranet or internet. Three years later, Internet Engineering Task Force (IETF) released Session Initiation Protocol (SIP) which essentially governs the same thing as H.323. The combination of all these digitisation technologies is what we now call Voice Over IP.

VOIP call centre

VOIP call centre is essentially the use of VOIP technologies in running a call centre. There are 3 main components, which we will discuss below: Customer Relationship Management (CRM) software, IP Private Automatic Branch Exchange (PABX), and VOIP infrastructure

Customer Relationship Management (CRM)

This is the principal piece of software used by the call centre agents. CRM itself can be a big software suite, of which only some parts are used by the agents, as discussed below. As we will see, CRMs tend to cater for the functionalities of inbound call centres better than outbound.

Computer Telephony Integration (CTI) middleware

This is the key component in the usage of CRMs in call centres

  • In inbound call centres, CTI enables the CRM to automatically show purchasing and interaction history whenever an existing customers dial in. This allows the agents to interact knowledgeably with the existing customers
  • In outbound call centres, CTI dials the potential or existing customer’s telephone numbers. This saves the agents’ time because they don’t have to dial the telephone numbers manually and also restricts the agents from calling people other than those who are supposed to be contacted

Unfortunately, how computers control the telephone systems is not uniformly standardised yet. There are still few different ways to perform this control, among them Computer-Supported Telecommunication Applications (CSTA) and Telephony Application Programming Interface (TAPI) etc. Even within the same scheme, there are multiple versions, with certain  degree of backward compatibility. As a result, for the CRM to be able to operate with as many PABX brands as possible, each might be speaking different language, the CRM usually employs CTI middleware. The CTI middleware translates the command from the CRM to whichever CTI scheme that the PABX can understand.

3rd party call control CTI in VOIP call centre

Activity management

Any inbound or outbound call should be recorded as activity against customer, case, or lead. This enables very detailed tracking of customer interaction history with the company

Lead management

Used only in outbound call centre, this feature provides the following:

  • list of potential customers to be called
  • regular distribution of potential customers to tele-sales officers. Few CRMs provide good lead distribution feature
  • latest potential customers’ response with regard to the offer
  • updated contact details of the potential customers, if the initial contact information is inaccurate
  • history of interaction and outcome
  • conversion of sales lead to sales opportunities once the potential customer agrees to the offer

Call scripting

Used only in outbound call centres, this standardises the verbal interaction between tele-sales officers and potential customers. Few CRMs provide a call scripting feature that’s interactive enough for this purpose

Order management

Used only in inbound call centres, this provides the customer service agents with purchasing history of the existing customers, to enable the agents to handle the customers’ requests better

Case management

Used only in inbound call centres, this feature provides:

  • list of complaints the customers have had
  • interaction history with regards to the complaints
  • ability to escalate the handling of more difficult complaints to higher level supervisors
  • latest status on the resolution to the complaints

Knowledge base

This provides answers to common customer questions or resolutions to common customer problems. It helps shorten the handling of the case, which makes customer happy and saves agents’ time

Application Programming Interface (API)

Photo by Flavio Takemoto

CRMs usually can’t cover Sales Order (SO) processing comprehensively enough. So, this business process is usually taken care at greater detail in the company’s Enterprise Resource Planning (ERP) software , which is the multi-department integrated software that forms the back bone of the company’s computerisation. So, 2-way integration has to be performed in the case of VOIP call centre:

  • sending qualified lead from CRM to ERP for SO processing
  • sending SO progress, including payment status, back from ERP to CRM

To facilitate this, the CRM should have an Application Programming Interface (API) which enables external applications like ERP to perform data processing inside the CRM remotely and correctly.

Internet Protocol Private Automatic Branch Exchange (IP PABX)

It is apparent that a call centre will have multiple connections to the outside world so that they can make or receive multiple calls at the same time. It is the job of the PABX to allocate connections to agents needing any. An IP PABX’ main function is to bridge voice connection between internal and external parties, regardless of whether the connection on either side is regular telephone or VOIP.

Switch board for Asterisk open source PABX
Connection requirements Internal (extensions) External (trunks)
Regular telephone Analogue mode only. FXS (Foreign Exchange Station) either in multiple single ports (RJ11) or single multi-port (RJ21) adapters Can be analogue and/or digital mode.
For analogue mode, FXO (Foreign Exchange Office) either in multiple single ports (RJ11) or single multi-port (RJ21) adapters.
For digital mode, single or multi-port E1/T1/ISDN adapters
VOIP Intranet Internet

Computer Telephony Integration (CTI) server

This is a very important role in VOIP call centre. It receives call request from and sends incoming call notification to the CTI client wrapped within the CTI middleware in the CRM. Please note that some CTI scheme like TAPI may require the CTI server to be separate from the IP PABX.

Interactive Voice Response (IVR)

Photo by Ivan Prole

This is used only in inbound call centres. Customers call for all sorts of purposes. The best way to handle this is to divide the customer service agents into multiple specialties. This way, the agents can build intimate knowledge in the subject of his/her specialisation in shorter time, allowing them to be very effective earlier. Once agents have been assigned specialties, there needs to be a mechanism to classify the purpose of the customer call, in order to be able to direct it to agents with correct specialties. Doing this manually will require as many dispatchers as there are connection  lines, not a cost-effective approach. The solution is IVR, where a pre-recorded message asks customer of the purpose of the call in form of multiple choice. The customer doesn’t answer the questions verbally though, as it would need a human interpreter. Instead, the customer answer by pressing the number buttons on the telephone. It is possible to interpret this answer by machine because of the invention of Dual-Tone Multi-Frequency (DTMF) in 1950s, which represents any number dialled at the telephone with a certain sound. Once the purpose of the call has been identified, the call can be directed to group of agents with matching specialties.

In countries where Calling Line Identification (CLID) has not been thoroughly implemented yet, IVR may also be used to identify the customer.

Automatic Call Distribution (ACD)

This is only used in inbound call centres. ACD assigns new customer call to customer service agent who is ready and isn’t handling any call. An agent has to inform ACD whether he/she is ready to receive the call by logging into an ACD queue. Whenever the agent is not ready, e.g. on toilet break or lunch break etc, then he/she has to log out of the ACD queue so that no calls will be pushed to him/her by the ACD. In case when all the agents are busy, ACD will put the customer on hold until a ready agent is free to receive the call

Call recording

Photo by Mihai Andoni

Some countries mandate the recording of conversations happening in call centres. The recording may be used as evidence in case there’s a future dispute regarding the circumstances of the call. Aside from the recording process itself, the process of finding and replaying a certain call recording should be easy enough to use. And from technical point of view, there are few things to consider:

  • What digital audio format should the recording be kept in?
  • Proper storage sizing should be done upfront to ensure adequate capacity to retain call recording for time period deemed necessary

Supervisory functions

A large call centre team should be divided into multiple groups, each with their own supervisor. One of the function of the supervisor is to coach the team members in performing their jobs. To assist in this, the PABX should provide the following functions:

  • monitor, where supervisor can listen in on the conversations without being heard by the agents or the customers
  • whisper, where supervisor can listen in on the conversations, and utter suggestions to the agents, without being heard by the customers
  • barge, where supervisor take active part in the conversations, turning them into 3-party conference calls

Call Data Record (CDR)

All calls, incoming or outgoing, should be listed by the PABX for future analysis

Asterisk open source PABX stores its CDR in database table for easy analysis

VOIP infrastructure


While the CRM server and the IP PABX will of course be located on the server farm, the call centre workstations should be placed under separate access switch stack. This is because they will generate large data transfer when transmitting VOIP and should be kept separate from workstations outside call centre

Internet Protocol (IP) phones

Photo by James38

The IP phones can be:

  • physical (hard-phone), which means it’s something the agent can pick up and touch
  • virtual (soft-phone), which means that it is a software residing within the agents’ workstation. Agents use it by way of head set connected to the sound card of the workstation

Regardless of existence of physical presence, internally, they work in exactly the same way. Make sure that the VOIP signalling protocol used by the IP phone and the IP PABX are the same though. An H.323 IP hard-phone will not work with SIP-only IP PABX, for example.

There are few things we can do to the phones to make call center agents’ job easy. One thing is to set up the phone to automatically answer any call within certain number of rings. Providing telephone head set also makes continuous calling comfortable for the call center agents. Choosing head set with noise cancellation microphone, although more expensive, makes conversation with potential or existing customers better as the customer doesn’t hear the noisy background sound of the busy call centre.


Thin client may be used in conjunction with IP hard-phone. Photo by Tommy Karlsson

The specification of the agents’ workstation depends on whether IP hard-phone or soft-phone is in use

  • If IP hard-phone is in use, then the workstations can simply be thin clients. It is cheaper per unit than Personal Computer (PC), but requires additional terminal server, and generate even more data traffic in terms of input and graphical output update
  • If IP soft-phone is in use, it is advisable to use PC for agents’ workstation as the voice signal processing will have to be done quick enough by the workstations themselves

VOIP provider

There are multiple scenarios on the usage of VOIP in call center to customer connections:

  • No external VOIP provider. This means that the VOIP technology is only used within the confine of the call centre. No telephone cost saving can be realised in this scenario
  • Long-distance VOIP provider. Here, the provider bridge the long distance part of the phone call using VOIP, lowering the cost of long-distance calls
  • Up to last mile VOIP provider. Here, the communication leaves the premise via data connection, not telephone connection. It goes to the VOIP provider in pure IP packet and routed via internet by the VOIP provider to local telephone network nearest to the customer. This provides maximum cost savings

Beside the cost saving, the main factor to consider here is the capacity owned by the VOIP provider. If the capacity is low, then it might result in bad voice connections, which is not good for the company image in the ears of its customers


3 thoughts on “Anatomy of VOIP call centre”

  1. Very good and thorough covering of VoIP as implemented in enterprises’ call centers, man.

    Be interesting if you could share the implementation details 🙂

    Btw, on the technology side, there’s another set of codecs that’s used to bring voice over the wire besides G.711 that is not so efficient for use on top of IP.

    All the best.


    1. Thanks, Rief. We are currently in the process of replacing our outsourced outbound call centre with in-house technology as well as planning for integrated outbound-inbound call centre technology stack. On the outbound side, we bought an outbound-friendly CRM but use open source for everything else (Asterisk IP PABX, Activa TAPI Service Provider for Asterisk, X-Lite soft-phone, etc). On the inbound side, we use our own ERP (you know which one), which has a mini-CRM inside it, plus the same open source stack as our outbound call centre.
      Paul Pambudi


  2. The integration of VOIP is, in my opinion , one of the main steps towards extended functionality for a call center, the other main step being decentralization. I.e. f you want to have your call center running 24 hours a day and attend customers around the globe, it is no longer necessary to have a stationary call center with a central server network anymore, thanks to technical progress. You can now build up a decentralized network through cloud computing that makes all required data acessible to agents anywhere anytime. This is only the beginning of a constant development into this direction.


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