Motivation:
How many times has it happened to you that your cell phone
was either off or in no-reception area/mod; an important
call came in, and you wouldn’t even know about it ?! How
annoying!
(Remember: currently missed calls notification works only if
the missed call(s) arrived when your phone was ON and in a
reception area/mod)
The feature:
Under certain
conditions, such as No answer or No registration, the SIP
network will forward incoming missed calls to a Unified
messaging server. Later on when the user shows “signs of
life” the Unified messaging server will notify the user for
the missed calls.
How this can be accomplished:
Figure 1 below illustrates how this can be accomplished.

Figure 1:
Missed Calls Notification service (Taken from our eLearning '72 reasons to
believe in SIP')