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Voice over Internet Protocol (VoIP)

VoIP's ability to converge voice, video and data into one application makes available new accessibility options not possible previously. Emerging VoIP applications are allowing blind and partially sighted people to use voice enabled applications for access to newspapers and magazines; hearing impaired people to use video telephones and text to speech applications to communicate; cognitively impaired people to use their voice to command their world and physically impaired people to use their work telephone number from home.

Other terms used include: IP telephony, internet telephony, voice over broadband (VoBB), broadband telephony and broadband telephone.

What is VoIP?

VoIP allows users to make telephone calls using a computer network, over a data network like the internet. VoIP converts the voice signal from the user's telephone into a digital signal that travels over the internet then converts it back at the other end so they can speak to anyone with a regular telephone number.

VoIP can be implemented in several ways, from a software program (softphone) on a computer used with a headset and microphone to adaptors used with regular analogue home telephones or a user can use dedicated VoIP telephones which look and act like regular telephones but have specially designed chips to improve call quality.


  • 1.8 million homes are now using their broadband connections for internet telephone calls [Ofcom, August 2006]

How does VoIP work?

Just like a modem converts digital signals from the computer into analogue (voice) traffic for transmission over a telephone line, a VoIP-enabled telephone or VoIP adaptor converts the user's voice into digital packets (using the special Session Initiation Protocol - SIP) for transmission over the internet. If the user makes a call to another VoIP telephone, the opposite process occurs at the other end and their voice emerges intact from the ether. If they make a call to a mobile or landline, their VoIP providers gateway decodes the call and sends it on as an ordinary voice call to the telephone exchange (Public Switched Telephone Network - PSTN).

The fact that VoIP transmits voice as digitised packets over the internet means that it has the potential to converge with other digital technologies, which in turn will result in new services and applications becoming available.

What is needed to make a VoIP call?

A VoIP enabled telephone

This can be an all-in-one handset, or a normal handset plugged into an adaptor, or a softphone: a computer program that uses a microphone and headphones attached to a computer to emulate a real handset. The softphone can be installed on a variety of hardware devices including wireless devices such as a Personal Digital Assistant (PDA).

An internet connection

A leased line or ADSL/cable broadband is ideal; dial-up (ISDN or ordinary telephone line) or a satellite internet connection will likely cause a reduction in sound quality. A standard 256Kb ADSL connection can accommodate 2-3 simultaneous calls; if a user expects to need more frequently, they may wish to upgrade to a higher-bandwidth package.

An account with a VoIP service provider

This gives the user a VoIP external number which is what other people dial to call them.

Mobile and PC VoIP

While the majority of VoIP services, such as Skype and Vonage, act like a normal landline, there are a range of other options for making VoIP calls over the internet.


  • Bebo, the social networking site, generates more than half (56%) of all UK visits to Skype, [Hitwise, June, 2006]

VoIP on a mobile

VoIP is still mainly limited to use at home, but some VoIP providers are teaming up with mobile networks to offer VoIP calls on the go. Notably, Skype is now available on some mobile handsets, including a range of 3's mobiles and the Apple iPhone.

Barablu offers mobile VoIP calling on any internet-enabled mobile telephone. Users download Barablu software to their mobile and can then call other Barablu users free, or normal landlines and mobiles cheaply.

A variation on VoIP is offered by websites like Jajah. These offer free or cheap calls between normal landlines or mobiles by redirecting calls through the internet. The user will need to be near a computer with an internet connection for it to work, because to make calls they will have to type in their number and the number they are calling on Jajah's website. Many calls are free once the user is registered with the service, but they will have to pay if they exceed Jajah's fair usage limit.

VoIP using instant messaging services

Some instant messaging services also offer the ability to make VoIP calls over the internet, though typically they only offer the option to ring other computers with the same instant messaging service as the caller.

The most popular of these is Windows Live Messenger, though GoogleTalk and Yahoo Messenger also offer VoIP voice calling.

Problems encountered by disabled people and the ageing population using VoIP

Potential accessibility barriers include:

Blind and partially sighted

People with visual impairments may face problems reading from the small or hard to read displays of some VoIP hardware telephones.

They may also face problems with some VoIP softphones that are not compatible with screen readers, in particular those that use moving text methods. Also, some VoIP softphones are not compatible with screen magnifiers and high contrast screens. Graphics-rich screens of some VoIP softphones may be hard to navigate through and read.

Hearing impaired

Reduction in audio quality may reduce intelligibility for people who are hearing impaired. Also, loss of synchronisation between audio and video in IP videotelephony reduces their intelligibility during speech-reading.

Softphone use on a computer may also cause interference.

Physically impaired

Physically impaired people may not be be able to use some VoIP hardware telephones due to hard-to-use controls, including touchscreens.

Cognitively impaired

People with an intellectual impairment may have difficulties managing softphone installation and configuration may be hard to understand. Some VoIP softphones have graphics-rich screens that may also be hard to understand.

Ageing population

Some members of the ageing population may experience a range of difficulties with VoIP, such as those stated above: from not reading the display of a VoIP hardware telephone; being unable to use some VoIP hardware telephones controls and being unable to hear or understand clearly.

Checklist for VoIP


Possible recommended features include:

  • Higher bandwidth will permit better audio quality, lip reading as well as sign language communication resulting in improved intelligibility

VoIP hardware telephone

  • Telephone should be ergonomically designed
  • General layout of the telephone should be standardised and functions should be easy to understand
  • Menu systems should be helpful and logical
      • Good contrast should be provided i.e. for text, graphics
      • Provision for adequate and adjustable illumination of the display whenever possible
      • A large tilting display makes it easier to see the picture clearly and can reduce the glare from lights
      • Anti-glare provision to avoid reflections whenever possible
      • A line by line presentation is often preferable to a single line of scrolling text
      • Combinations of blue, green and violet should be avoided
      • Colour alone should not carry information
      • Visual indication of line status is available
      • Visual indicator for ringer, or a socket for external flashing light
    • Keys
      • Good visual contrast between the keys and the body of the telephone
      • Key tops should be convex or flat with a raised edge
      • Keys should be as large as possible without reducing the distance between the keys to less than half the key width
      • Ideally the keys should be internally illuminated, but the internal illumination should not reduce the legibility of the numbers in daylight
      • The visual markings on the keys should be of high contrast, clear and as large as possible on the key top
      • Keys should be raised above the body of the telephone (preferably by 5 mm)
      • Auditory and tactual feedback of key activation
      • Tactual indication on the '5' key or on a QWERTY keyboard on the 'F' and 'J' keys
      • A voice mode selection that announces all key presses
      • Key functions that are easy to understand
    • Fonts
      • Fonts should be clear, have a minimum size of 12 point, with high contrast backgrounds
      • There is clear letter spacing between each character
      • Kerning between specific characters is sufficient so as to ensure legibility
      • Avoid the use of italic fonts as they are more difficult and slower to read
      • Avoid patterned backgrounds for fonts
      • Ensure the font has both upper and lower case letters as it is easier to read text in upper and lower case than all capital letters
      • Ability to select larger text
      • Information should be presented in small amounts which can be easily retained
    • Audio
      • There is the ability to select special volume settings
      • Sound indication of line status is available
      • High audio quality
      • There is an option to switch off verbal feedback when/if not required
      • There is an option to switch off non-speech sounds, if used
      • Voice warnings should be presented in a voice that is different from other voices that will be heard in the task situation
      • Maximise the intelligibility of the messages
      • Make the voice as natural as possible so people are more likely to accept it
      • If the message is missed, it is beneficial for people to be able to replay it
      • If the message is familiar, the ability to interrupt the message would be beneficial for experienced users
      • Where the choice of messages is relatively limited, human voices are preferred because synthetic speech is less intelligible and less preferred
      • Use non-speech audio messages only for the purposes of alerting
    • Touchscreens
      • The touchscreen should be shielded from sunlight
      • The screen should be angled towards the horizontal to provide arm/wrist support
      • The screen should be perpendicular to the line of sight
      • The text and background colour combination should have high contrast
      • Avoid shades of blue, green and violet for conveying information since they are problematic for older users
      • There should be no noticeable flicker on the screen
      • Structure the visual display layout so that the user can predict where to find required information and how to use it

VoIP softphones

  • Compatible with screen readers
  • A line by line presentation is often preferable to a single line of scrolling text
  • Compatible with screen magnifiers
  • Non graphics-rich screens allowing for easy navigation
  • Reduce the interference that may arise by using a softphone on a computer
  • Installation is simplified and configuration is easy to understand
  • Product documentation is available and in alternate formats
  • Customer support is made available

Relevant standards

  • ITU-T F.724 (2005) Service description and requirements for videotelephony services over IP networks
  • ITU-T F.733 (2005) Service description and requirements for multimedia conference services over IP networks
  • ITU-T V.150.1 (2003) Modem over IP networks: Procedures for the end-to-end connection of V-series DCEs
  • ITU-T V.151 (2006) Procedures for end-to-end connection of analogue PSTN text telephone over an IP network utilizing text relay
  • ITU-T V.152 (2005) Procedures for supporting Voice-Band Data over IP networks
  • ITU-T Y.1541 (2006) Network performance objectives for IP-based services
  • TIA-1001 (2004) Transport of TIA-825-A signals over IP networks

Further information