Sent messages are not delivered sometimes

There could be several reasons:

The carrier of the receiver doesn't provide delivery reports;
Some technical issues on the side of the receiver’s carrier (like overload);
The message delivery to device was blocked by the receiver’s carrier due to some internal reason;
The destination phone number is not correct, not reachable, listed in the stop lists or does not exist;

Some information we feel will be useful for you:: — What is the message length?; — Can I send and receive messages with Unicode?; — I do not receive messages sometimes; — Sent messages are not delivered sometimes; — Formatting international phone numbers; — Sender ID; — Acceptable Use Policy