When sending SMS messages via direct connections to mobile operators, there should be no failures in the delivery of messages when subscribers are available to receive SMS. Very rare, isolated cases can most often occur during technical work on the equipment of the operators themselves, problems on communication lines, as well as when subscribers are roaming.
In most cases, problems with the delivery of messages occur when you send SMS through connection to transit (indirect) operators, delivery is not guaranteed due to the fact that some ultimate mobile operators can block spam filters, bulk SMS in their networks, sent via data feed to transit operators.
Can also be a problem with the display of SMS in the phone when sending a composite message from multiple SMS parts if for any reason one of the parts of the message have not been delivered to the end device of the subscriber, for example, when the buffer is full SMS. In this case, the phone may not display the message, and the status in the system can be set to "Delivered" as determined by the last part.
Most often, with the received status "Delivered", messages may not be displayed in the phone if there are problems related to the hardware or software part of the subscriber's phone itself, for example, in the case of incorrect firmware installed in the phone, the use of third-party programs for working with SMS, some anti-spam programs or antivirus programs. In such cases, the hardware part of the phone can receive the message and return a notification of successful acceptance to the operator, but then the software part may not display the SMS to the subscriber. In such a situation, you can check by turning off the phone and sending an SMS again. The message must accept the status "Passed to the operator", and this status must not change. If this status changes after turning on the phone, it means that the device still received the SMS, but did not display it.
Another reason for not displaying messages may be the lack of support for the alphabet specified in SMS, for example, there is no support for the Russian language. This can be verified by sending in transliterated or English letters.
With incorrect firmware, some phones, for example, devices based on the Android system, may stop displaying messages when internal indexes fail, associated with a large number of messages in the SMS history, although they notify the subscriber of the arrival of the SMS with the appropriate sound signal. In such cases, deleting old messages and restarting the phone helps.
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