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Reasons Why Some Messages Don't Get Delivered

1) The recipient's phone is switched off

If you get a busy signal while calling, you try again later. But when the SMS doesn’t get delivered on the first try, it is automatically re-sent in certain intervals. When the phone is available again, the message gets delivered. If a certain amount of time passes without the SMS being successfully delivered, your mobile carrier might decide to stop trying.

2) 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. These problems are usually resolved quickly, but they can slow down your message delivery quite significantly.

3) The number might be in the Do Not Disturb list

In some countries, the operators have a list, where handset owners can add their phone number by themselves so no messages from an A2P connection can be delivered.

4) The handset settings have "receive SMS messages" turned off

These days, SMS messaging isn't the only way that people receive text messages. 

5) Conflicting SMS applications

Sometimes different applications on the handset can receive SMS messages.