You will hear “unreachable” when the paging of the MS fails, and the paging can fail even if the MS is on. The paging can fail due to a number of factors, such as congestion on the paging channel.
However, when the paging fails (MS unreachable), a simple VAS application can instruct the SMSC to send a text message to the mobile, notifying it of the missed call. The implementation can be done in one of 2 ways.
1. The SMSC sends the message immediately, and keep trying until it reaches the subscriber( remember that with an SMS, the system can keep trying for a longer time, and for several more times, than a call).
2. The SMSC sends the text message the next time the mobile makes contact with the network (eg, location update, call initiation, etc.)
In conclusion, I’ll say that you will always have a number of these cases…because it’s impossible to ensure a 100% paging success rate.