PBX - VOIP - Telephone

Greyhound Technologies

PBX & Telephone Services

PBX functions

Functionally, the PBX performs four main call processing duties: Establishing connections (circuits) between the telephone sets of two users (e.g. mapping a dialled number to a physical phone, ensuring the phone isn't already busy) Maintaining such connections as long as the users require them (i.e. channelling voice signals between the users) Disconnecting those connections as per the user's requirement Providing information for accounting purposes (e.g. metering calls) In addition to these basic functions, PBXs offer many other calling features and capabilities, with different manufacturers providing different features in an effort to differentiate their products. Common capabilities include (manufacturers may have a different name for each capability):

  • Auto attendant
  • Auto dialing
  • Automatic call distributor
  • Automated directory services (where callers can be routed to a given employee by keying or speaking the letters of the employee's name)
  • Automatic ring back
  • Call accounting
  • Call Blocking
  • Call forwarding on busy or absence
  • Call park
  • Call pick-up
  • Call transfer
  • Call waiting
  • Camp-on
  • Conference call
  • Custom greetings
  • Customised Abbreviated dialing (Speed Dialing)
  • Busy Override
  • Direct Inward Dialing
  • Direct Inward System Access (DISA) (the ability to access internal features from an outside telephone line)
  • Do not disturb (DND)
  • Follow-me, also known as find-me: Determines the routing of incoming calls. The exchange is configured with a list of numbers for a person. When a call is received for that person, the exchange routes it to each number on the list in turn until either the call is answered or the list is exhausted (at which point the call may be routed to a voice mail system).
  • Interactive voice response
  • Music on hold
  • Night service
  • Shared message boxes (where a department can have a shared voicemail box)
  • Voice mail
  • Voice message broadcasting
  • Voice paging (PA system)
  • Welcome Message

Interface standards

Interfaces for connecting extensions to a PBX include: POTS (plain old telephone service) - the common two-wire interface used in most homes. This is cheap and effective, and allows almost any standard phone to be used as an extension. proprietary - the manufacturer has defined a protocol. One can only connect the manufacturer's sets to their PBX, but the benefit is more visible information displayed and/or specific function buttons. DECT - a standard for connecting cordless phones. Internet Protocol - For example, H.323 and SIP. Interfaces for connecting PBXs to each other include: proprietary protocols - if equipment from several manufacturers is on site, the use of a standard protocol is required. ISDN PRI - Runs over T1, 23 bearer channels + 1 signalling channel QSIG - for connecting PBXs to each other, usually runs over T1 (T-carrier) or E1 (E-carrier) physical circuits. DPNSS - for connecting PBXs to trunk lines. Standardized by British Telecom, this usually runs over E1 (E-carrier) physical circuits. Internet Protocol - H.323, SIP and IAX protocols are IP based solutions which can handle voice and multimedia (e.g. video) calls. Interfaces for connecting PBXs to trunk lines include: standard POTS (plain old telephone service) lines - the common two-wire interface used in most domestic homes. This is adequate only for smaller systems, and can suffer from not being able to detect incoming calls when trying to make an outbound call. ISDN - the most common digital standard for fixed telephony devices. This can be supplied in either Basic (2 circuit capacity) or Primary (24 or 30 circuit capacity) versions. Most medium to large companies would use Primary ISDN circuits carried on T1 or E1 physical connections. RBS - (Robbed bit signaling) - delivers 24 digital circuits over a four-wire (T1) interface. Internet Protocol - H.323, SIP, MGCP, and Inter-Asterisk eXchange protocols operate over IP and are supported by some network providers. Interfaces for collecting data from the PBX: Serial interface - historically used to print every call record to a serial printer. Now an application connects via serial cable to this port. Network Port (Listen mode) - where an external application connects to the TCP or UDP port. The PBX then starts streaming information down to the application. Network Port (Server mode) - The PBX connects to another application or buffer. File - The PBX generates a file containing the call records from the PBX. The call records from the PBX are called SMDR, CDR, or CIL. It is possible to use a Voice modem as FXO card.

Hosted PBX systems

A hosted PBX system delivers PBX functionality as a service, available over the Public Switched Telephone Network (PSTN) and/or the internet. Hosted PBXs are typically provided by the telephone company, using equipment located in the premises of the telephone company's exchange. This means the customer organization doesn't need to buy or install PBX equipment (generally the service is provided by a lease agreement) and the telephone company can (in some configurations) use the same switching equipment to service multiple PBX hosting accounts. Instead of buying PBX equipment, users contract for PBX services from a hosted PBX service provider, a particular type of application service provider (ASP). The first hosted PBX service was very feature-rich compared to most premise-based systems of the time. In fact, some PBX functions, such as follow-me calling, appeared in a hosted service before they became available in hardware PBX equipment. Since that introduction, updates and new offerings from several companies have moved feature sets in both directions. Today, it is possible to get hosted PBX service that includes far more features than were available from the first systems of this class, or to contract with companies that provide less functionality for simple needs. In addition to the features available from premises-based PBX systems, hosted-PBX: Allows a single number to be presented for the entire company, despite its being geographically distributed. A company could even choose to have no premises, with workers connected from home using their domestic telephones but receiving the same features as any PBX user. Allows multimodal access, where employees access the network via a variety of telecommunications systems, including POTS, ISDN, cellular phones, and VOIP. This allows one extension to ring in multiple locations (either concurrently or sequentially). Supports integration with custom toll plans (that allow intra company calls, even from private premises, to be dialed at a cheaper rate) and integrated billing and accounting (where calls made on a private line but on the company's behalf are billed centrally to the company). Eliminates the need for companies to manage or pay for on-site hardware maintenance.

Mobile PBX

A mobile PBX is a hosted PBX service that extends fixed-line PBX functionality to mobile devices such as cellular handsets, smartphones and PDA phones by provisioning them as extensions. Mobile PBX services also can include fixed-line phones. Mobile PBX systems are different from other hosted PBX systems that simply forward data or calls to mobile phones by allowing the mobile phone itself, through the use of buttons, keys and other input devices, to control PBX phone functions and to manage communications without having to call into the system first.


An IP PBX handles voice signals under Internet protocol, bringing benefits for computer telephony integration (CTI). An IP-PBX can exist as physical hardware, or can carry out its functions virtually, performing the call-routing activities of the traditional PBX or key system as a software system. The virtual version is also called a "Soft PBX".

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