The PBX may use this information to route the call directly to a telephone extension within the organization without the need for an operator or attendant.
The central office equipment detects the level of the line and disables service if the circuit is not operational.
This is the reverse arrangement from standard plain old telephone service (POTS) lines for which the central office provides signaling and talk battery.
In the United States the feature was developed by AT&T in the 1960s, patterned upon the earlier IKZ service of the Deutsche Bundespost in Germany.
In theory, standards such as T.38 should have allowed VoIP subscribers to keep their existing fax equipment working locally; in practice, T.38 at the subscriber's site offers no benefit if the upstream provider is least-cost routing to gateways that do not support T.38 and cannot reliably send or receive fax/modem traffic.
A fax server at a central location, connected directly to public switched telephone network (PSTN) T- or E-carrier primary rate interface lines and using direct inward dial to identify the intended addressees can convert incoming faxes to electronic documents (such as TIFF or PDF) for web or e-mail delivery.
The fax traffic never passes through the VoIP infrastructure as a dial-up modem call and therefore arrives reliably even if T.38 is not properly supported at some points in the network.