When an SMS is sent to a Telecommunications Service Provider, they return a delivery receipt. Telecommunications Service Providers send the delivery receipt at a moment of their choice, not actually having to wait for delivery confirmation. A delivery receipt can be either: Carrier – returned to the message deliverer when the message is received by the Telecommunications Service Providers. Handset – returned to the message deliverer when the message is received on your user’s handset. In practice, some Telecommunications Service Providers either do not send the delivery receipt or send a fake delivery receipt . Unfortunately no message deliverer can be 100% certain that a ‘successfully delivered’ delivery receipt actually means that the message reached the user. If the message is not received, this delivery receipt explains why the message failed to arrive. See SMPP Delivery Receipts [smpp.org] .
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