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. In other words, 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.
This call service is used when, after initiating a call to the called party, the calling party is unable to
receive a response, or the VoIP device of the called party denied the call request from the calling
party. 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.
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 will still be established through another voice subscriber line in the hunt
Call barring includes incoming call barring and outgoing call barring.
Incoming call barring usually refers to the Do Not Disturb (DND) service. When incoming call barring
is enabled on a voice subscribe line, calls originated to the attached phone will fail.
When outgoing call barring is enabled on a voice subscriber line, calls originated from the attached
phone will fail, too.
Message waiting indication
The message waiting indication (MWI) feature allows a voice gateway to notify a subscriber of
messages received from a voice mailbox server. For example, when a call destined to subscriber A is
forwarded to the voice mailbox server, the server will notify the state change to the voice gateway.
When subscriber A picks up the phone, subscriber A will hear the message waiting tone without
needing to query the mailbox.
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, thus implementing
three-party conference. For example, subscribers A and B are in a conversation, and subscriber C is
the supervisor. If C wants to join the conversation, it sends a request to A. If A permits, the