From SIPfoundry sipx, The Open Source SIP PBX for Linux - Calivia

[edit] Defining a ring down address

A ring down address is used to automatically dial a pre-defined number every time the phone is taken off hook.

PHONESET_RINGDOWN

Enables or disables the “ring down” feature for a device. By default, the ring down feature is set to DISABLE. To enable this feature, store a SIP URL as the PHONESET_RINGDOWN_ADDRESS and select either:

• IMMEDIATE: Automatically dial the ring down address when creating a new call for the purpose of dialing
• PROXY: Collect dialed digits, then try the ring down address. When connected, play the dialed string as DTMF tones

For example, to configure a phone that is located in the lobby of a facility so that anyone who takes the phone off hook will automatically connect to the facility switchboard, you configure the phone as follows:

PHONESET_RINGDOWN : IMMEDIATE
PHONESET_RINGDOWN_ADDRESS : sip:operator@<domain>