SLD Security & Communications - state of the art digital communication systems
IP Telephone Systems


With Voice-over-IP, you'll know that your voice and the dialling information is all transferred digitally across your network and the Internet. However the concept of a switchboard (exchange) is still vital. It is still the job of an exchange to keep track of subscribers (location and status) and route calls to and from them. Of course, as PBXs (and public exchanges) developed, many additional services were introduced, such as caller ID, call diversion and call waiting - all of these are managed and operated by the exchange/PBX.

Another important change with IP PBX's is that locally, within your premises, you can use IP phones - the call is carried over your existing data network all the way to your phone, so the phone plugs into your standard RJ45 socket on the wall (you can use WiFi too). With a traditional PBX, every extension had to be wired directly back to the PBX, whereas with an IP PBX, you have just one network cable from the PBX into your existing Ethernet switch, and another network cable to your Internet connection. Obviously, you may have more connections if you choose a hybrid PBX which combines IP and traditional phone/lines connections.

In summary, a PBX (whether IP, ISDN or analogue), can perform the following:

  • Monitor the Status of a subscriber (busy, absent, free)
  • Time and log all calls made (for statistical or billing purposes)
  • Take voicemail (answerphone) messages in case of busy/no-answer
  • Store the voicemail for telephone retrieval or forward the message by email
  • Route calls from extensions appropriately (to the right destination or interface)
  • Accept incoming calls and pass them to the appropriate extension
  • Transfer calls between extensions
  • Conference Calls
  • Call Waiting
  • Caller ID

What is an IP PBX ?

An IP PBX is a telephone switchboard which uses IP (the Internet protocol) to carry telephone calls over computer networks instead of using the PSTN (the regular phone network) and traditional phone lines (ISDN or analogue). Telephone extensions on a traditional PBX are connected using regular analogue connections (phone sockets) with each socket being wired back individually to the PBX. An IP PBX doesn't use phone lines (analogue/ISDN) to connect to the PSTN but instead uses the Internet (or sometimes a private IP network) to connect to a PSTN gateway (IP Trunk).

An IP network uses your existing Ethernet (CAT5/UTP) within your premises. Once your IP PBX is installed onto your network, your IP telephones can communicate with the PBX by plugging into any of your network sockets, in the same way that you can plug in any computer. You do not need specific extension sockets so installation is much simpler and more flexible as you can move an extension just by plugging it into a different socket (an extension's identity is determined by the phone's settings, not the socket it is plugged into).

IP, being the protocol of the Internet can be passed directly onto the Internet using your broadband line or other Internet connectivity so if you have an IP phone plugged in somewhere else on the Internet (anywhere in the world) that phone can connect to your PBX and call to/from the IP phones in your office.

VoIP Trunks

Of course, until the whole world uses IP, most of your calls will still be to or from the regular telephone network (PSTN) so you need a method to call to/from your IP network. On a pure IP PBX, there are no analogue or ISDN trunk lines; the IP PBX has no direct connection to the PSTN, just a connection to the Internet. You therefore need to use a PSTN gateway - this is a server which interconnects your IP network with the PSTN so that your outbound calls can be passed onto any regular phone number and you can have a regular PSTN phone number which routes into your PBX. This service, or provision is called a VoIP Trunk (or a 'SIP' trunk as SIP is the signalling protocol used).

The following two diagrams show the topology (the connection relationship of all components) in firstly a traditional PBX and then an IP PBX:

PBX Schematic

Above is traditional PBX. For every line, you need another connection into the PBX from the outside world, so line rental costs and for every extension, you need another dedicated cable all the way from the phone back to the PBX. This means a lot of wiring, and therefore difficult to change later. Traditional PBXs are also generally much larger physically.

IPPBX Schematic

With an IP PBX (above) you use your existing computer network, so only one connection is required back to the PBX. Splitting anywhere along the way with additional Ethernet switches as required. A DSL trunk (your connection to the outside world) replaces all of your multiple incoming lines.

We supply, install and maintain a range of IP telephony hardware for the small business, with systems supporting up to 30 telephone extensions (local or remote) and with up to 20 simultaneous external calls in progress at any one time.

IPPBX 2820 Integrated IP-PBX & ADSL Router

  • Voice-over-IP PBX
  • ADSL Firewall/Router
  • 802.11n WiFi
  • Up to 30 VoIP Extensions (local or remote)
  • Up to 6 SIP Trunks & 20 Channels
  • FXO Port (Analogue Line Interface)
  • Voicemail with personal user messages
  • Remote Access to Voicemail
  • Trunk/SIP Budget controls
  • T.38 Fax support on analogue extension
  • Hunt Groups
  • Call Diversion/Transfer
  • IVR (Voice Auto-Attendant)
  • Two ISDN2e Interfaces (4 channels)
  • Analogue Phone Interface (with POTS failover)

IPPBX Pricing
Image of Item VPBX2820N VigorIPPBX 2820N
Combined WiFi Router and IP PBX (Office Phone System)
Image of Item T18PN Yealink T18PN IP Phone £45
Image of Item T22PN Yealink T22PN IP Phone £85
Image of Item T26PN Yealink T26PN IP Phone £100
Image of Item T28PN Yealink T28PN IP Phone £129
Image of Item EXP38 Yealink EXP38
38-Key BLF Expansion module for T26PN and T28PN
Image of Item YHS32 Yealink Headset
Fits T22PN and T26PN
Image of Item YPSUUK Yealink PSU
For T18PN, T22PN, T26PN and T28PN
Required if not using PoE
Image of Item VSP1080 VigorSwitch P-1080 PoE Ethernet Switch
8-Ports, all fully powered, full/half power PoE.
Ideal for IP Phones
Image of Item POEINJ1 PoE Injector
Provides power to IP Phones

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