Reasons for SMS Delivery Failure

Our mobile phones sometimes inform us that a text message hasn’t been delivered and it will retry later. It can be frustrating, but now imagine the problem for an enterprise sending hundreds, thousands or millions of  messages to their customers. A frustration now becomes a significant business challenge. This begs the question:

How do SMS delivery failures occur and what can be done to mitigate these failures?

The list below sets out the six most common reasons for message delivery failure and the preventative measures that can be taken.

1. Telephone Number is Invalid

It sounds obvious, if a telephone number is invalid then the message will not be delivered. However, testing if a number is valid prior to sending a message requires specialised number validation services that perform multiple complementary checks.

Explore how XConnect enables customers to validate numbers and compare it with other different approaches: Comparing Number Validation Solutions.

2. Messaging Spam

While SMS spam is usually unwanted in some markets it is also unlawful. Many service providers in the delivery chain take measures to reduce spam. These solutions aren’t perfect and if tuned too aggressively can block valid messages.

You can read more about how the industry is fighting to reduce spam and distinguishing between spam and valid messages: Mobile Ecosystem Forum: Fraud Management.

3. Coverage

SMS service providers rely on partnerships to maximise their connectivity to the world’s approximately 900 mobile operators. It is not uncommon for some operators or countries to be missing and coverage can vary from month to month. Larger service providers can have more consistent reliable coverage where messages are relayed through fewer partners.

The more partners in the chain the harder it is to assure delivery: Mobile Ecosystems Forum: Trust in Enterprise Messaging.

4. Number Portability

To maximise the chances of successful delivery, service providers must deliver each text message to the correct operator. In most markets, consumers can keep their number and change operators, known as number portability. In some markets, messages that are misrouted are forwarded to the correct one. In others, misrouted messages are simply rejected.

Therefore, service providers must use a service like XConnect’s Number Portability Lookup to find out who the operators are in real time prior to routing the messages.

5. Handset Issues

If a phone is switched off or out of coverage, messages can’t be delivered. However, the network is designed to store undelivered messages and try again later. To help the sender understand if the messages have been delivered, delivery receipts (DLRs) are returned to the sender. The problem is that the propagation of the routed, relayed or delivered status back through each of the message transit points is unreliable. This unreliability is a result of different network technologies, coverage, and processes along the route. Therefore, it is difficult to determine if a message has failed due to a handset issue or another issue. Service providers that adopt industry best practices will have enhanced DLR capabilities.

The Mobile Ecosystems Forum (MEF) published the A2P SMS Features Guide highlighting the best practices. 

6. Roaming

If a subscriber is not on his/her home network, some service providers will not be able to guarantee delivery. This is because they choose to route via the roaming network rather than routing via the home network and relying on the home network to relay the message. This is usually done to minimise the cost of delivery. Routing via the roaming network can also be challenging because it is difficult to obtain routing information. Service providers would normally find this routing information through an HLR Lookup. However, due to security and data protection issues, this information is now protected and only available to a controlled group of partners. For further information, see Why HLR Lookup is not a viable method for enabling your SMS and voice routing decisions.

About the author – Lee Suker, Market Development Director & Data Protection Officer




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