A telephone VoIP adapter (TVA), also called digital telephone adapter, is a device that allows digital PBX (Private branch exchange) telephone handsets, e.g., Nortel Meridian, and BCM or Siemens Optiset/Optpoint, to be connected to a VoIP network, using, for example, SIP (session initiation protocol).
An ATA (analog telephone adapter) allows analog phones to be connected to VoIP networks.
A Centrex TVA VoIP enables centrex phones, such as Nortel P-Phones, and analog-based Centrex phones like BT Featureline.
Some telecom manufacturers have produced hybrid exchanges with TVA-like elements that support IP phones and also have units or cards that allow connection of digital phones. For instance Avaya IP Office release 7.0 supports expansion modules for Avaya's digital phones, or alternatively "heritage" Nortel Norstar/BCM phones.
Whether a standalone TVA (such as Citel's Portico TVA) or a hybrid PBX is deployed, the intention is to preserve investment in an installed base of telephone handsets, and remove the need to install Ethernet network infrastructure.