The National Relay Service
The National Relay Service (NRS) is a national telephone access service available to everyone at no additional charge.
(HCO) - hearing carry-over service, enables callers with a speech or communication impairment to hear their what their caller says and type their responses, which are then read to their caller by the Relay Officer.
(VCO) - voice carry-over service, which allows callers to speak directly to the other party as the relay officer listens to the conversation and types the responses which appear on the teletypewriter;
(SSR) - the speech to speech relay service, where a person with a speech impairment can use their natural voice or a voice output device to speak directly to the other person and relay officers will respeak parts of the conversation as necessary.
The NRS is provided by Australian Communication Exchange (ACE), a not for profit company, under a contract with the Commonwealth: http://www.aceinfo.net.au
A PhoneChoice consumer information article