SMS Delivery – What are The Reasons for SMS Nondelivered & How to Increase SMS Delivery Rate

Why are SMS messages not delivered?

If you came to our website with the aim to send mass SMS on your subscriber’s database, everything seems very easy from first sight. The work process at the SMS-Smart platform is really simple – register, upload contacts database, start an SMS campaign.

But when going into details, bulk SMS may become an issue for SPAM in case a bulk SMS campaign is created incorrectly, as actually, you are sending the same SMS texts to a huge number of phone numbers. Besides, sometimes you input some URL into the SMS text, which is also may seem suspicious for the network operator which is accepting SMS for further transfer to the mobile number of the subscriber.

First of all, let’s clarify the process of sending and receiving SMS behind the scenes of SMS-Smart. What happens after you click the button SUBMIT CAMPAIGN?

So, when you are clicking the button, all your SMS texts go to the network switch, it extracts the number of the SMS center and tries to send the message to it. SMS-center is like a gateway between the sender of SMS and its receiver.

What Is SMS-Center and What is it Responsible For?

SMS-center analyzes the fields “From whom” and “To” and on their basis decides whether to accept this message or reject it. Upon its solution, the switch retransmits the message to the sender, who is waiting for confirmation all the time. After receiving the answer, the SMS-Smart system displays the status – for example, “sent” or “delivered”, “rejected”, etc. In such case SMS-center, having received the message, saves it in the database and periodically tries to find the recipient and deliver the message to him.

If the response is not received within a certain time (usually 48 to 72 hours), the device considers that the transmission failed, the status may change to “expired”.

SMS-center is a software and hardware complex designed for receiving, registering, and sending short messages. SMS-center, like any other network device, has its own unique identifier – number. There can be many SMS centers in one network.

What are the General Reasons for the Low Delivery Rate?

Let’s move through the potential reasons for SMS isn’t being delivered

  1. The phone number does not exist or the phone number has an invalid format;
  2. The subscriber can not accept SMS (for example, if SMS service is not connected, the subscriber is in roaming, where the reception of SMS is not activated, the subscriber’s operator does not have an SMS exchange with the current roaming operator, or it is a city phone number without the ability to receive SMS messages).
  3. The subscriber is out of network coverage at the time of SMS arrival;
  4. The subscriber is blocked (if the subscriber has zero or negative balance, roaming, intentional blocking by the subscriber, damage to the SIM card, incorrect pin code, etc.);
  5. The subscriber’s device does not support receiving SMS;
  6. The common queue at the SMS-center of the mobile operator is overloaded;
  7. The queue for sending SMS to a specific subscriber is overloaded (it may occur when the subscriber is not active/available for a long time);
  8. The subscriber is busy;
  9. Receiving of SMS is prohibited by the subscriber (the subscriber may require the operator to prohibit sending him advertising and bulk SMS, like DDN numbers – do not disturb number);
  10. Sending SMS at an unsuitable time (if the corresponding settings of the permissible time of sending SMS are made in the personal account);
  11. The limit of messages to the subscriber is exceeded (it may occur when a lot of messages are sent to the subscriber in a short time);
  12. No route (may occur when there is no available route to send SMS to the specified number);
  13. The number does not belong to any operator;
  14. Technical failure on the side of the operator;
  15. Technical failure on the side of the SMS provider;
  16. The mobile device or the phone number gives the status “Delivered”, but does not display the message;
  17. Incorrect operation of the mobile device of the subscriber;
  18. Subscriber’s phone memory is full;
  19. An error without a reason from the network operator, etc.

All these possible reasons either on the side of the SMS channel side, or the SMS accepting side can be displayed by the system in different ways. So, you may meet the following SMS delivery status inside the system.

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SMS-Smart Statuses Overview (Delivery Report Statuses)

  • DELIVERED – your SMS has been delivered successfully
  • SENT – means SMS was sent, but not delivered yet, this status might change on delivered if the phone number is active and available, otherwise – the status will change for expired – which means – SMS could not reach the phone number.
  • REJECTED – the reason for such a status may be: the wrong format of the phone number (having 10 digits instead of 11). Some other reasons for such status are: SMS was sent but rejected by SMS-center because of one of the following reasons: the network is either out of our coverage; as the number is not recognized due to an incorrect number prefix or number length; due to the user being subscribed to DND (Do Not Disturb) services, disabling any service traffic to their number;
  • UNDELIVERED – the message was sent but the system received a delivery receipt indicating that the message was not delivered. This can happen for a number of reasons including carrier content filtering, availability of the destination handset, etc.
  • UNKNOWN – means the number unreachable, invalid or not in service.
  • EXPIRED – status means that the SMS center failed to deliver SMS to the subscriber after a number of attempts during a long period of time.
  • NOT ACCEPTED – this a very rare issue, which is connected with the system settings, so please immediately cont
  • act us for support.

Dear customers, if you have these statuses and what to clarify the reasons for it, please contact us. We can make a more deep investigation and find ways to make your SMS campaign the most effective.

How to Increase the Delivery Rate of Your SMS Campaign?

Follow the instructions and recommendations by your SMS service provider. In many countries, there are special requirements and restrictions, time limits, and schedules for the bulk SMS  not to be recognized as SPAM. For example, in China or Saudi Arabia, all the SMS are controlled by the government, and all local operators strictly filter and block suspicious SMS texts.

Possible Recommendation to Follow for Successful Bulk SMS Campaign:

  • Pay attention to the “from” field. It is shown to the subscriber on his mobile phone when he receives an SMS. It is the sender ID you are using while creating an SMS campaign. If you want to use the name (Alfa sender ID) you need to contact your SMS service provider to register sender ID. Otherwise, all your SMS will some other sender ID or numeric sender replacement. Besides the maximum length of text sender ID is 11 symbols, all sender IDs which are longer will be blocked, and SMS not delivered.
  • In the US, only numerical Sender IDs are allowed and all A2P SMS needs to be sent from a text short code, whereas in the UK it can be alphabetical.
  • Pay attention to the recipient’s phone number (must contain the country code BUT doesn’t contain “+” sign)
  • Pay attention to the message body. Search on the web which is the stop words in SMS. It may be “credit card”, “bonus”, or even URL in the body of the message may become a subject for filtering.
  • Pay attention to the message length and encoding.  The maximum character length for 1 SMS in Latin-based alphabet languages like English, French, and Spanish is 160 symbols. Long text messages are divided into parts 152 symbols each. The maximum length of 1 SMS by other non-English characters (languages not using a Latin-based alphabet, like Arabic or Chinese) is 70 symbols. Long text messages are divided into parts of 67 symbols each.
  • Pay attention to the local network restrictions. Countries like Brazil only support SMS of up to 157 characters of length. If your SMS length is over this limit, it will be rejected if you while setting the SMS campaign or your SMS service provider doesn’t automatically divide it into multiple SMS (see Long message split mode in SMS-Smart). In China, there are time restrictions regarding promo SMS delivery.
  • Regarding the encoding, based on the GSM specification, the limit is 160 symbols for GSM 7bit, 140 symbols for GSM 8bit, and 70 symbols for GSM 16bit.
  • Divide your SMS campaign into parts. Sometimes when there is a huge number of SMS (100K and more), SMS are put into a queue at the SMS center. In order to avoid this queue send your bulk SMS by parts, schedule the campaign (there is an option to schedule an SMS campaign at SMS-Smart). For this, divide your phone numbers into groups, attach tags to each group, and, for example, first send to 5K numbers, then to 10K and 10K. Besides, send SMS to these groups at intervals of 30 minutes.

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How to Solve the Problem of Non-Delivery of SMS?

Please contact us in case you are facing any problems with SMS-Smart dashboard operation, set up, sending SMS, or setting SMS campaign. Once you’ve done everything yourself – that’s great!

If you checked the Statistics and noticed your SMS has Not Accepted status, rejected, not delivered – please contact us for investigation and details.

We are always ready to help!

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