High Availability (HA) refers to methods used to ensure that a system can reliably and continuously serve users as intended. HA uses design protocols and implementations to ensure a high level operational continuity over a specified time period.

Availability refers to the ability of users to access a system. If a user cannot access the system, it is considered unavailable. The term downtime is often used to refer to a period when a system is unavailable.

VoIP systems can experience unavailability during periods of very high usage, system maintenance, or device failure. HA for VoIP often involves the use of various methods such as load balancing, automatic failover mechanisms (e.g servers using Virtual Router Redundancy Protocol or VRRP), designed or built-in live upgrading and patching capabilities, multiple data storage, backup power, and hardware redundancy to achieve very high levels of service availability.

The two main factors influencing high availability in a VoIP environment are capacity and redundancy. Capacity refers to the volume of traffic a network can handle. Voice networks are usually designed to handle a peak-load capacity measured in calls per second. Redundancy refers to the extra or reserve capacity that a network has available in the event of failure or downtime of network equipment.

The Service Availability Forum was established by the communications and computing industry to promote the creation of HA network infrastructure products, systems, and services. HA design principles have been used in fields aside from computing such as nuclear power, aeronautics, and health care.