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subscriber C (final recipient). After Subscriber A hangs up, the call between subscriber B and
subscriber C is established. This is call transfer.
To perfect the call transfer feature, the device supports the call recovery function after the call
transfer fails, that is, if subscriber C in the previous example is in a conversation with another
subscriber and cannot establish a conversation with subscriber B, the call between subscriber A and
subscriber B is recovered.
Call backup
After initiating a call to the called party, the calling party is unable to receive a response. In this case,
if there is another link (PSTN link or VoIP link) to the called party, the calling party re-initiates a call to
the called party over the new route. This is call backup.
The system supports two types of call backup:
• A PSTN link or VoIP link backs up a PSTN link.
• A PSTN link backs up a VoIP link.
Hunt group
Multiple voice subscriber lines are configured with the same called number to form a hunt group. If
the voice subscriber line with the first priority is unavailable when a call setup request to the called
party is received, the call is still established through another voice subscriber line in the hunt group.
Call barring
Call barring includes incoming call barring and outgoing call barring.
Incoming call barring usually refers to the DND service. When incoming call barring is enabled on a
voice subscribe line, calls originated to the attached phone fails.
When outgoing call barring is enabled on a voice subscriber line, calls originated from the attached
phone will fail, too.
Message waiting indication
The MWI feature allows a voice gateway to notify a subscriber of messages got from a voice mailbox
server. For example, when a call destined to subscriber A is forwarded to the voice mailbox server,
the server notifies the state change to the voice gateway. When subscriber A picks up the phone,
subscriber A hears the message waiting tone without needing to query the mailbox.
Three-party conference
When subscriber A has a call with subscriber B and holds a call with subscriber C, A can make C join
the current conversation to implement a three-party conference.
During a three-party conference, a passive participant can initiate a new call to create another
conversation. In this way, conference chaining is implemented, and each conference initiator serves
as a conference bridge.
Silent monitor and barge in services
Silent monitor service—Allows a supervisor to monitor active calls without being heard.
Barge in service—Allows a supervisor to participate in a monitored call to implement three-party
conference. For example, suppose subscribers A and B are in a conversation and subscriber C is the
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