Why are my outgoing SMS not being delivered?

Non-delivery means we were unable to deliver the message. There may be multiple reasons for failure to deliver, examples being:

Issue Explanation
The number format is not correct All numbers need to be added in the international format. For more information see here: Numbers format.
The phone was switched off Make sure the mobile number you enter is available and valid so that you do not waste your credit unit on messages sent to unachievable phone numbers.
The network did not enable receiving SMS for this subscriber (e.g. due to low balance, or a do-not-disturb (DND) list for SMS services)
The phone was out of network coverage Most connection problems are resolved simply by restarting the phone because the connection to the network is refreshed.
There was an error in the Application to Person (A2P) SMS connection between the phone and the network. For further information, please see below (Difference between A2P and P2P)
The number is in roaming (outside of the home country) We can not guarantee delivery for roaming numbers. This depends on the agreements that we can conclude between the home network and the roaming network.
The messages were not delivered due to local restrictions. Many countries and networks have restrictions regarding SMS content and senders. For detailed information per country, please see Country Info & Restrictions.
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