■■ Incoming and outgoing call screening—Incoming and outgoing call screen-
ing can be implemented in either a proxy or a user agent. ARequest-
URIor Fromheader is compared to a list of allowed or blocked URIs,
and an appropriate response generated, such as 403 Forbidden, in the
event of a call being blocked. The outgoing call screening feature can
only be implemented in a proxy if the user agent is configured to
always use that proxy as an outgoing proxy. The incoming call-screening
feature can only be implemented in a proxy if the user agent is config-
ured to accept only requests from an incoming proxy, redirecting all
other requests with a 305 Use Proxyresponse.
■■ Automatic callback and recall—Automatic recall allows a PSTN user to
return a missed call based on calling line identification. This is easily
implemented in a user agent by caching the Fromheader from the pre-
vious failed INVITErequest. Automatic callback allows a PSTN user
whose call fails because of a busy signal to have the call automatically
placed, as soon as the called party becomes free. This can be imple-
mented in a SIP network using a simple presence service, in which a
SUBSCRIBEis sent to request notification when the called user agent is
no longer busy. The NOTIFYresponse would then automatically gener-
ate a new INVITEto complete the call. This was shown as an example
in Figure 6.11 in Chapter 6, “SIP Overview.”
■■ Speed dial—Speed dial allows a user to place a call by dialing a shorter
digit string, often stored in the network or in the telephone set. A SIP
user agent can use any speed dial method. Alternatively, a mapping
from a “nickname” to a full URI is possible to allow easier “dialing” in
a similar way that nicknames are useful in e-mail.
■■ Conference calling—Conference calling is described in Chapter 14, “SIP
Conferencing.”
■■ Voicemail—This important service is described in Chapter 12, “Voice-
mail and Unified Messaging.”Table 11.3 Types of Call Transfer
TRANSFEROR TRANSFER CONSULT NEW TALK TO BOTH
ACTION PARTY PARTIES
Unattended Yes No No
Consultation Hold Yes Yes No
Attended Yes Yes Yes198 Chapter 11