Meaning of SMS delivery reports
When sending SMS messages, we show and overview of message delivery status displaying the number of messages delivered successfully, undelivered or buffered (not yet delivered). This article is an attempt to clarify the meaning of these status reports.
These are messages that were passed on to the operator and were successfully delivered to recipients' phones. This status report actually indicates message receipt on the subscriber's phone, with some exceptions like the United States, Canada and some other countries where it indicates successful delivery to the mobile operator. In Europe, Asia, Africa, South America it is the actual receipt on the phone, though.
If this delivery report has been received but the user claims not having gotten the message, it is overwhelmingly likely that the issue is on the phone-side: blocked sender, disabled notifications, or even phone software glitches.
This is an intermediate status that appears when the message has been sent but we haven't yet received a final delivery status. This status can be displayed for quite a long time in situations when the mobile operator cannot immediately reach the recipient and attempts message delivery at a later time until the message expires.
Some of the common reasons are:
Phone switched off;
Phone outside network range;
Full memory, although that is a rarer issue in modern smartphone age;
Prepaid card balance empty
In all these cases the message delivery has encountered a temporary error that possibly could resolve within the message expiry period, so the operator holds off on discarding the message and reporting as undelivered.
The message expiry period, after which the operator will give up and report an undelivered message is by default 24 hours, although it can be set to a shorter period if necessary. While some phones allow you to set a very long message validity time, with very rare exceptions, periods longer than 24 hours aren't supported for A2P (application-to-person) messaging.
This is another final message status that indicates that the message could not be delivered successfully to the recipient's phone, or even to the operator. As there is only a single mode of success but many of failure, this is a longer list than others.
While we attempt to deliver every single message our clients have entrusted us, it is an unfortunate fact that out of the messages you send not every one of those will be delivered to your customers. The most common reasons for failed delivery are:
Number currently not reachable: the most often of these reasons, this indicates that the number is valid and assigned to an existing customer, and an attempt to deliver this message was made over the period of message validity (usually 24 hours) but it was ultimately unsuccessful. The reasons for this might be that the recipient's phone is out of network coverage, switched off.
Incorrect number: while we check if recipient numbers are correctly formed, and in locales where it is possible, the number is assigned to a valid operator, sometimes numbers are by all appearances well-formed but just not correct, and your customer will not receive their message. In this case the mobile operator might report something like unknown subscriber or SMS service not provisioned to us. In this case the undelivered message status will be reported immediately.
Older number: the number might have been working when the customer signed up but they have let it lapse and it is no longer in use by the mobile operator. In this case the undelivered message status will be reported immediately.
Full memory: this is less of an issue these days but it is possible in older phones, in phones where SIM card storage for SMS is used, or rarely extremely full smartphones, where the device simply doesn't have enough free memory to accept an incoming message. In this case the mobile operator retries delivery over the message validity period (usually 24 hours) in case some memory is freed up during that time, and the phone can finally accept the message.
Repeated messages: Identical messages that are being sent in a short period of time, that is, messages with the same content, recipient number, and sender ID will be blocked if identical message was sent less than 5 minutes ago. This is to prevent automated messaging failures where, for example, due to some error some server keeps sending the same message over and over in a loop, and not overwhelm the recipient with too many messages.
Unsubscribed recipients: These messages will be blocked in our system and will not even be passed to the mobile operators. These are numbers who have unsubscribed to receiving messages from your sender ID.
As the mobile operators don't have a standardized way of reporting message failure reasons, we only show a single undelivered status to avoid being misleading. Some operators report have very granular error codes for most conceivable reasons, and some don't report them at all. If, however, there is an important reason to get this information, you can always contact our support and we'll check what data is available.
Delivered
These are messages that were passed on to the operator and were successfully delivered to recipients' phones. This status report actually indicates message receipt on the subscriber's phone, with some exceptions like the United States, Canada and some other countries where it indicates successful delivery to the mobile operator. In Europe, Asia, Africa, South America it is the actual receipt on the phone, though.
If this delivery report has been received but the user claims not having gotten the message, it is overwhelmingly likely that the issue is on the phone-side: blocked sender, disabled notifications, or even phone software glitches.
Buffered
This is an intermediate status that appears when the message has been sent but we haven't yet received a final delivery status. This status can be displayed for quite a long time in situations when the mobile operator cannot immediately reach the recipient and attempts message delivery at a later time until the message expires.
Some of the common reasons are:
Phone switched off;
Phone outside network range;
Full memory, although that is a rarer issue in modern smartphone age;
Prepaid card balance empty
In all these cases the message delivery has encountered a temporary error that possibly could resolve within the message expiry period, so the operator holds off on discarding the message and reporting as undelivered.
The message expiry period, after which the operator will give up and report an undelivered message is by default 24 hours, although it can be set to a shorter period if necessary. While some phones allow you to set a very long message validity time, with very rare exceptions, periods longer than 24 hours aren't supported for A2P (application-to-person) messaging.
Undelivered
This is another final message status that indicates that the message could not be delivered successfully to the recipient's phone, or even to the operator. As there is only a single mode of success but many of failure, this is a longer list than others.
While we attempt to deliver every single message our clients have entrusted us, it is an unfortunate fact that out of the messages you send not every one of those will be delivered to your customers. The most common reasons for failed delivery are:
Number currently not reachable: the most often of these reasons, this indicates that the number is valid and assigned to an existing customer, and an attempt to deliver this message was made over the period of message validity (usually 24 hours) but it was ultimately unsuccessful. The reasons for this might be that the recipient's phone is out of network coverage, switched off.
Incorrect number: while we check if recipient numbers are correctly formed, and in locales where it is possible, the number is assigned to a valid operator, sometimes numbers are by all appearances well-formed but just not correct, and your customer will not receive their message. In this case the mobile operator might report something like unknown subscriber or SMS service not provisioned to us. In this case the undelivered message status will be reported immediately.
Older number: the number might have been working when the customer signed up but they have let it lapse and it is no longer in use by the mobile operator. In this case the undelivered message status will be reported immediately.
Full memory: this is less of an issue these days but it is possible in older phones, in phones where SIM card storage for SMS is used, or rarely extremely full smartphones, where the device simply doesn't have enough free memory to accept an incoming message. In this case the mobile operator retries delivery over the message validity period (usually 24 hours) in case some memory is freed up during that time, and the phone can finally accept the message.
Repeated messages: Identical messages that are being sent in a short period of time, that is, messages with the same content, recipient number, and sender ID will be blocked if identical message was sent less than 5 minutes ago. This is to prevent automated messaging failures where, for example, due to some error some server keeps sending the same message over and over in a loop, and not overwhelm the recipient with too many messages.
Unsubscribed recipients: These messages will be blocked in our system and will not even be passed to the mobile operators. These are numbers who have unsubscribed to receiving messages from your sender ID.
As the mobile operators don't have a standardized way of reporting message failure reasons, we only show a single undelivered status to avoid being misleading. Some operators report have very granular error codes for most conceivable reasons, and some don't report them at all. If, however, there is an important reason to get this information, you can always contact our support and we'll check what data is available.
Updated on: 05/03/2021
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