VOIP is an acronym for Voice over Internet Protocol, or in more common terms phone service over the Internet. If you have a reasonable quality Internet connection you can get phone service delivered through your Internet connection instead of from your local phone company.

PBX (Private Branch Exchange) is a system that connects telephone extensions of a company to outside public telephone network as well as to mobile networks. An IP (Internet Protocol) PBX is a PBX that provides audio, video, and instant messaging communication through the TCP/IP protocol stacks for its internal network and interconnects its internal network with the PSTN (Public Switched Telephone Network) for telephony communication.

IP Phones are the same thing as VoIP Phones or Soft Phones. These are telephones that use VoIP technologies for making calls over an IP Network or the traditional PSTN networks.

SIP (Session Initiation Protocol) is a signaling communications protocol, widely used for controlling multimedia communication sessions such as voice and video calls over IP networks.

IAX (Inter-Asterisk Exchange) is a communications protocol native to the Asterisk PBX software, and is supported by a few other soft switches, PBX systems, and soft phones. It is used for transporting VoIP telephony sessions between servers and to terminal devices. IAX now most commonly refers to IAX2, the second version of the IAX protocol.

IAX2 protocol carries both signaling and media on the same port 4569 (by default). SIP signaling port is 5060 (by default), but media goes through other random ports. For IAX2 you only have to make sure port 4569 is clear then the phone calls can be guaranteed, but for SIP, besides port 5060 also other RTP ports need to be guaranteed too for stability of communication. Otherwise you’ll encounter no audio or one-way voice problems, especially between different networks. From the functionality perspective, IAX2 cannot support variety functions like SIP. SIP is the most widely used VoIP protocol so IP phone manufacturers are willing to develop call features based on SIP instead of IAX2.

T.38 is an ITU recommendation for allowing transmission of fax over IP networks in real time. Sidpex M200 IP PBX supports T.38 fax pass through.

Some IP Phones provide different ring tones based on call source (internal or external) and there’s text field to enter Alert-Info so that distinctive ring can be used for that destination.

DISA (Direct Inward System Access) is a mechanism available on some PBX’s that permits inbound calls to be answered and immediately presented with system dial tone. The caller is then able to dial a number that the PBX uses to decide how to forward the call. It is like the caller is able to dial twice – first to reach the PBX, then a second time to reach the final destination using the facilities of the PBX.

Define patterns to match the numbers you call, then those numbers will be stored into Asterisk database and be associated with the extensions which made the calls. If those numbers call in again, the calls will automatically call on the extensions that associated previously.

Phone provisioning or auto provisioning is an easy and time-saving way to configure IP phones for IP PBXs. With phone provisioning, all user information can be entered via the IP PBX web interface. Required is the MAC address of the IP Phone, the desired extension and the caller ID which is displayed on the called party phone display. The IP Phone receives the configuration via the local network. This offers many advantages, i.e. it is possible to configure phones centralized, which saves a lot of time and money.

1. Single Network: This is one of the biggest advantages of of IP Telephony systems. Traditional PBX require their own network and it is quite expensive to build a separate telephone network spanning the entire premises. With IP Telephony, the Computer Network can be used to carry voice calls along with data and the IP Phones connect directly to Network Switch Ports using Cat 5/6 Cables. Most of the IP Phones come with an inbuilt two-port switch that eliminate additional switch ports and cables required for their connectivity. Redundancy is an issue, and the IP Phones stop working if the computer network is down, but redundancy can be built into IP networks using technologies.

2. Inter-branch Calls: If IP Telephony has been deployed in multiple branches (in different locations) of the same company, it is possible to use MPLS Networks/ Internet Leased Lines (With Unlimited Usage plans) to transmit voice calls over the WAN IP Network. This way, the inter-branch calls would not incur additional costs.

3. Long Distance Calls: Its possible to terminate SIP Trunks from ITSP’s (Internet Telephony Service Providers) directly to the IP PBX. So, international calls and long distance calls can be made through the Internet for lower cost. Concepts like DISA (Direct Inward Station Access) allow a user to dial to the corporate IP PBX (from anywhere using a PSTN Phone/ GSM Cell Phone) and access the IP Trunks connected to it, to make long distance calls at reduced rates.

4. Easier Management: The Analog/ Digital PBX is difficult to manage. Some of them can only be managed using complex CLI Commands that are proprietary to each vendor. But an IP PBX generally has a web based GUI (Graphical User Interface) console to manage/ configure/ make changes to many of its functions. This makes it easier for administrators to manage an IP PBX. Users may even be given a custom web-page which they could use to login and set their own preferences.

5. Soft-Switch: Some IP PBX models come as down-loadable software that can run on standard computer servers. These are called Soft-Switches. Soft Switch based IP PBX have a lot of advantages. There are some open-source based Soft-switches that can be downloaded free of cost (Like Asterisk, Elastix, Trixbox, FreePBX, etc).

6. Cell Phone/ Land Line Integration: You can download a SIP Client on your Cell Phone (that supports this feature) to receive land line calls on your cell phone itself, through a Wi-Fi Network. So, your cell phone can become your mobile land-line extension!

7. IP Phones/ Soft Phones: IP Phones may be costlier than analog phones, but IP Phones have a lot of advantages over analog phones like easy movement from one place to another (while still retaining the extension number), Connecting to Internet using inbuilt browsers, Down-load ring-tones, etc. IP PBX support Soft-Phones that are software programs which can be run on a computer and they have a number of advantages over IP Hard-phones. These soft-phones can be used along with headset/mic to receive all your land-line calls directly from your desktop PC.

8. Encryption: IP PBX / IP Phones are capable of encrypting conversations by using techniques like sRTP (Secure Real Time Protocol). Though this technique is not used often, it can be used to deter hackers from listening to voice calls over the IP Network.

9. Fax Server: IP PBX support IP Faxing which, among other things, can be used to receive and send faxes directly from a computer.

10. Analog Trunks/ Analog Phones: Analog/ Digital Trunks like ISDN / PRI Lines / FXO Lines etc from the Telephony Service provider can be directly terminated on an IP PBX. Similarly, Analog Fax Machines and Analog Phones can also be connected to an IP PBX. Both use devices called Analog Telephony Adapters (ATA). The main advantage with ATA is the fact that it can be present anywhere on the network. For example, if an Optical Fiber Cable connects to an individual department, the ATA can be placed in that department to connect the FXO/FXS trunk and subscriber terminals directly, while communicating back to the IP PBX over the IP Network (Using the Optical Fiber Cable).

11. Video Calls: With an IP PBX, it is possible to make video calls along with audio calls over the IP network. There are Video Phones that can be used for this purpose, and some IP PBX vendors support this functionality.

12. Call Recording: Often, you may want to record certain voice calls for future reference. In analog/digital PBX, a separate line needs to be connected from each phone (in parallel) to an expensive equipment called Voice Logger. But with certain IP PBX models, call recording is an inbuilt function and can be activated by the user through their IP phone/ PC interface whenever required.

13. Presence/ Instant Messaging (IM): Some IP PBX models come with built-in Instant messaging function that can be used along with a PC based interface. This is similar to the web based IM that we are familiar with. The instant messaging function includes presence information which enables the caller to see if the user is available and their preferred mode of communication at that point of time.

14. Conference Bridge: Only simple built-in conference facilities are available with Analog/ Digital EPABX. Even that is restricted to 3-8 party conference calls at a given point of time. But IP PBX can enable a Meet-Me Conference (sometimes included as a default functionality) which allows many users to dial into a conference room to enable multi-conference calls. There can be multiple such conference rooms and multiple callers in each conference room. The users could even be given a security pass code that authorizes them to enter the conference call.

15. QoS & Voice Compression: Since IP Telephony shares the same network as computers and other IT equipments, the available bandwidth often needs to be shared between multiple type of devices. But fortunately, up to 10 Gigabit Ethernet bandwidth is available for the IP Network backbone today, which is sufficient for most enterprise applications. Further, Network Switches support QoS policies to be applied to IP Phones and voice related applications so that real time latency sensitive voice traffic can be given priority over data traffic. Various Voice Compression CODECS are available to compress voice signals over the IP Network.

16. Remote Maintenance: Both IP PBX and IP Phones can be accessed remotely (with sufficient authorization, perhaps over VPN Networks) for making configuration changes and monitoring purposes from anywhere over the Internet. The analog PBX might support remote access in a limited way, but analog/digital phones cannot be accessed from a remote location.

17. Voice Mail/ IVR: Though Voice Mail functionality is available in an analog/ digital PBX as well, there is a limitation on the number of hours of voice mail that can be recorded. IP PBX use Servers/ Computer based disks to store voice mail and hence have more voice mail storage capacity. An IP PBX can even send a notification email to the user when a new voice mail is recorded. Advanced Interactive Voice Response (IVR) creation / customization can be done relatively easier and the tree structure changed frequently using IP PBX.

18. Help-desk/ Call Center functionalities: Some basic call center/ help desk functionalities can be built in to IP PBX like Call Queuing, Group Ringing, Automatic Call Distribution, etc.

19. Database Integration: A Number of interesting applications can be enabled by integrating databases (like MySQL, etc) with IP PBX. An IP PBX can be programmed to fetch certain database entries, when requested by the users, by pressing certain key combinations guided by an IVR – Interactive Voice Response. Mobile Banking is a good example of an application enabled by IP PBX through Database Integration.

20. Application Programming Interface (API): External programs and applications can interface with IP PBX using the Application Programming Interface that is provided by many IP PBX vendors. For example, when a customer is ringing the help-desk of a certain company, the IP PBX recognizes that it is a regular customer (via the phone number), fetches recent orders and their current status and displays them on the screen of the help-desk employee. So, even before the employee picks up the phone, he/she might be acquainted with the required information to answer the call! This is enabled by integrating the Customer Relationship Management Application with an IP PBX using API.