Why SMS are not delivered?

Message Delivery Failures:

When an SMS message is published to iClosed, the message is forwarded to downstream providers. The downstream providers, in turn, forward the message to mobile phone carriers. The mobile phone operators are responsible for the last leg of message delivery to phone numbers. That is, the flow for SMS messages is as follows :

iClosed SMS Service -> Downstream providers -> Mobile Phone carriers -> Mobile EndpointSometimes issues in downstream providers and carriers can be the reason for the failures/delay of SMS notifications. That being said, the delay/failures mostly might have occurred due to one of the following reasons.

1. The phone number is temporarily out of the coverage area.

2. The phone number is in a roaming network.

3. There's increased network traffic for a particular carrier.

4. The phone was turned off when a carrier tried delivering the message.