When you call your friend and the phone is switched off or out of the carrier’s reach, you know that instantly.
With SMS, it is much more complicated, as most of the heavy-lifting is done without you knowing it. Here are the main reason SMS messages go undelivered.
1) The phone is switched off or out of the carrier's reach
If you get busy signal while calling, you can try again later or just give up. When the SMS does not get delivered on the first try, it is automatically re-sent in certain intervals without you knowing it. So when the phone is available again, the message still gets delivered.
Note: Messente's system is configured to try to deliver messages within 6 hours. When the message still fails, it is marked as 'Failed.'
2) False negative
Sending SMS messages als requires receiving Delivery reports (DLR) from carriers or operators. When the operator fails to respond with the correct receipt, the sender isn't notified of the delivery -that message actually got delivered - and marks it as 'Failed.'
NB! USA, Canada and most Latin-American countries are known not to provide delivery reports at all.
3) The number is roaming (abroad)
Sending an SMS to a number that is currently not in the home-country often causes lots of problems, and therefore, delivery to numbers that are roaming is not guaranteed.
4) The sender you are using is prohibited
Some countries or operators are blocking some types of senders or A2P messaging platforms. Japan, for example, does not allow sending A2P SMS messages from regular phone numbers. Telia Sonera in Finland, on the other hand, is blocking messages from alphanumeric senders like “CarService1″ or “BestHotel."
5) The carrier is blocking the SMS
Operators are also known to block some SMS routes from time to time. This also requires SMS services to re-route the message via another working route, which can cause temporary delivery issues.