Depositing of Voice Messages
Voice messages can be created for various reasons, such as if the called party is
not available, busy, there is no reply, or for other reasons. SIP can also support
such caller preferences where the caller prefers voicemail anyway to avoid
speaking to a party that would take up too much time. See the section “Prefer
Voicemail” in [3].
It is quite simple in a pure Internet environment to connect as caller to voice-
mail by using SIP Service Control based on the design of URIs for services [4].
If, for whatever reason, the SIP proxy or the SIP UA wants to direct the call to
voicemail, they can use the request URI to specify the destinations, as shown
in this example:sip:[email protected]In the user part of the SIP URI, instead of UserBonly, new target attributes
are specified as well: depfor message deposit and fbto indicate the reason
is forward on busy. Note the simplicity of this approach in a pure Internet
environment.
The design challenges are, however, more complex when compatibility with
TDM voicemail systems is required (such as for phone companies with legacy
TDM switch systems and in enterprises with TDM-based PBX systems). Espe-
cially in environments like call centers, the reason code and the so-called his-
tory of the call are important indicators. To meet the requirements in mixed
TDM and IP systems, a different encoding of the desired service of the SIP URI
has been proposed [5], where the SIP error code can indicate the reason for
retargeting the call. This type of usage opens more capabilities. Here is an
example:sip:[email protected];target=bob%40example.com;cause=486In this example, voicemailis the user part in the domain example.com
and the actual target is [email protected](where @is replaced with %40as
per the syntax for URI). The SIP reason code is 486 (user busy), and it is
called here “cause”. The causeparameter can have the values as shown in
Table 12.1.Table 12.1 Reason for Redirect Values in the SIP Request URI
REASON FOR REDIRECT VALUE
Destination Unknown/No available 404
User Busy 486
No Reply 408214 Chapter 12