An important aspect of the traditional telephone system is its consistent availability over time. Through decades of development, most APEC region PSTN networks yield an uptime of 99.999% - or less than six minutes of downtime a year.

Currently VoIP technologies do not guarantee a similar level of availability. Common threats to VoIP availability may be the result of any number of physical or electronic occurrences, which may be either malicious or accidental in nature. The following items detail some of these potential threats:

One of the primary reasons why VoIP does not perform as well as traditional telephone systems in availability is because it depends on computer networks and the Internet to work. The limitations of current communications technology mean that these computer network connections can only handle a certain level of data transfer – known as ‘bandwidth’ – at any given point in time. All Internet connectivity suppliers and broadband users are subject to bandwidth limitations. By adding VoIP to home or business networks, the bandwidth may be insufficient such that performance of both voice and data transmission suffers.

For calls within a single physical location, the VoIP call may not leave the internal network, however for calls to people outside the office or home, most VoIP systems travel over public computer networks. As a result, these systems may be susceptible to the full spectrum of computer network denial of service (DoS) risks. A large number of these DoS risks relate to the network or devices being ‘flooded’ with an unmanageable amount of traffic, such that the devices are unable to function normally. Other such attacks utilise weaknesses in the software or hardware components to exploit the devices and reconfigure them so they can no longer be accessed or used.

Traditional telephone lines have electricity provided from the telephone exchange. This means in the event of a power failure at an individual’s home or place of work, traditional telephone systems will often still be available for use. As most VoIP systems require electrical equipment (including modems, VoIP telephone systems, and personal computers) to be available in addition to the Internet connection, such telephone systems may not be available in the event of an electrical outage.

Inability to Access to Emergency Services

Emergency telephone numbers (000, 911 etc depending on your location) are not always available with VoIP services. It is up to the VoIP provider to ensure the access is provisioned. In cases where downloadable software is used to set up VoIP via an Internet connection, emergency services are quite often not available. In addition to this, the location information associated with your VoIP call may not be reliable enough for emergency services staff to locate you as quickly and easily as traditional telephone services, especially if an emergency call is made from a VoIP solution currently not at it’s ‘home’ location (e.g. a VoIP adapter / phone utilised interstate or overseas).

Certain premium rate call services and free call services may not be available. For example, service providers may not be able to provide call services to premium rate numbers such as those utilised in competitions, voting numbers, weather forecasts and other services. Free call numbers may also be non contactable. 

Voice Spam

Voice spam refers to the sending of unwanted or illegitimate calls on a VoIP network. Voice spam may also be referred to as Spam over Internet Telephony (SPIT). The key concern of spam in VoIP is the congestion it could cause to voicemail boxes, which unlike people, cannot identify and reject spam calls. Electronic stores where voice mail is kept could undergo significant load, and organisations may lose genuine voice mail messages. Fortunately voice spam is currently not a common occurrence but is forecast to grow in the future.

Another potential issue of voice spam is the impact it could have on the QoS of other legitimate calls. In extreme volumes, voice spam may congest bandwidth and reduce throughput for both data and voice services in an organisation.