How SMS texting works on UK mobile networks

When using any texting service, there will be instances where text messages don’t make it to the end user’s mobile phone. This post aims to help senders understand why this occurs.

Imagine a physical letter needs to be sent to a relative. There are two potential ways that this letter can be sent.

One option would be to use a courier company. You contact them and ask them to come and collect the letter and deliver it to the destination. A courier arrives, takes the letter from your hand, drives to the address marked on the letter and hands it over to the named person and gets them to sign for the receipt of the letter. You can then contact the courier company and ask to see the signature of the person who received the letter. If the letter is not delivered because the courier was involved in a street mugging, the courier company will be able to tell you about the incident, give you a crime number and confirm that the letter was not delivered.

The other option is to put a first class stamp on the letter and pop it into the nearest post-box. The letter then goes into a system with millions of other letters and passes through many people. It will be in a mail bag with hundreds of other letters, parcels and postcards and will eventually pop out of the system at the other end. It will be taken by the postman delivering the letters and parcels to the recipients’ addresses and items will be dropped through the letterbox without a signature. They may be delivered to the wrong address by accident (every so often we all get next door’s mail) or the home’s pet dog might find the post on the mat and gnaw on some of it. There is also a possibility that the letter sits in a sorting office for six weeks and is eventually delivered.

There is a number of differences between these services. The courier is much more expensive but has complete traceability, the first class stamp is much cheaper but it’s impossible to work out if a letter that doesn’t make it to the recipient was stolen from a mail bag in the great train robbery, lost in a street mugging of a postman or eaten by the family dog. About the only thing we do know about the second option is that the letter was put into a postbox.

All texting services rely on the UK mobile network system which is run by a number of companies: O2, Vodafone, T-mobile, Orange and a few others. The texting service in the support site allows an end user to compile a ‘letter’ in the form of a text message and then drop it into the ‘postbox’ which is an electronic interface onto the UK mobile network. This system is almost identical to the first class stamp and the postman and nothing like the courier service as the system does not have any traceability. The courier service would be like picking up the phone and calling the person you want to deliver the message to.

We do have a few tricks up our sleeve to deal with problems in this system. Quite often, the mobile numbers in a school MIS system are simply wrong (too many or too few digits) and the USO system will flag that to the Nominated Contacts in the school. We also get some information back from the mobile network. They can tell us if the destination mobile has responded in some way to the network as the message was delivered and this is then shown as ‘Delivered’ in the support site SMS logs.

A number of other bits of information come back from the network:

  • ‘Blacklisted’ – The recipient has added themselves to a list that prevents them from receiving any “bulk” messages. To opt back in to receiving such messages the person would need to re-subscribe to marketing messages from any company using a short 5 digit phone number.
  • ‘Expired’ – This means that delivery was attempted until the Validity Period was reached and the delivery failed. The Validity Period varies from one network to another and will depend on the recipient’s network provider.
  • ‘Number no longer in use’– The Networks believe the number is inactive. This number will be dropped back into the available numbers pot and reused at a later date as a new number.
  • ‘Number not recognised’ – The network does not recognise this number as one of its own. The owner of the number would need to speak with their mobile network to confirm that the number belongs to the network (for example, it might be a number ported over from a previous network and has somehow fallen off the radar).
  • ‘Phone Unavailable’ – Messages are undeliverable when the phone is out of network coverage or the recipient has switched the phone off.
  • ‘Recipient out of credit’ – The recipient’s phone is out of credit.
  • ‘SMS disabled by this person’ – The recipient’s number is disabled from receiving SMS texts. They would need to speak to their network operator to enable this service.
  • ‘Undeliverable’ – Messages are undeliverable when the phone is out of network coverage or the recipient has switched the phone off.
  • ‘Unknown’ – The message was delivered to the mobile network but we have received no feedback at all from the system.

Unfortunately, for the reasons outlined above, it is not possible to provide any further information with regard to a text message or how it has ‘done’ in terms of transiting the UK mobile network and being delivered to the end recipient.



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