Advantages of IP PBX when compared to Analog/Digital PBX (IP PBX Vs Traditional PBX):

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 like Link Aggregation, RSTP, etc.

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, 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. Fixed Mobile Convergence: As an enhancement to the above mentioned point, it is possible to automatically shift between Wi-Fi Networks and Cellular Networks by using a technology called Fixed Mobile Convergence. So, when you are attending a land line call on your cell phone using the Wi-Fi network and you suddenly move out of the office, the call can continue on a cellular network!

8. Wi-Fi Phones: Wireless Networks (Wi-Fi) are very popular, and can be found almost in every company. It is possible to use special Wi-Fi based Phones to attend to land line calls from where ever you are, within the company premises (Wi-Fi Zone). The DECT standard followed by Digital PBX also allows to do something similar, but a separate and a dedicated digital wireless network is required to enable the same. There are a lot of advantages of VoWLAN (Voice Over Wireless LAN) technology when compared to DECT (Digital Enhanced Cordless Technology).

9. 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.

10. 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.

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

12. 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).

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

14. Unified Communications: Unified Communications (Or UC) is an emerging field in IP Telephony that integrates multiple ways of communications like voice calls, video calls, voice mail, email, fax messaging, Instant Messaging, Cell Phones, etc and allows the user to use one mode of communication to communicate with other modes, seamlessly.

15. 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.

16. 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.

17. Speech Recognition: Speech Recognition, is another emerging technology that has successfully been integrated with IP PBX which recognizes voice input (speech) directly from a caller to perform a desired action. For example, a caller could call the board number of a company and say the name of the person whom he wants to talk to, for the IP PBX to fetch the extension number from a database and forward the call to that extension automatically.

18. Meet Me Conference: 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.

19. 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.

20. 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.

21. Hosted IP PBX: With an IP PBX, its possible to host the IP PBX Software (with a service provider) and register your IP Phones to the hosted IP PBX in their premises over the Internet. The service providers take care of the hardware/ software and maintenance of the hosted IP PBX, while charging a monthly fee for the Service.

22. 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.

23. 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.

24. 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.

25. 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

Source: http://www.excitingip.com/1945/ip-pbx-vs-traditional-pbx-advantages-of-ip-pbx-over-analogdigital-pbx/