Fix: Text message (SMS) Not Delivered

So your iPhone/Phone is unable send text messages - It shows a red exclamation mark (!) beside the message and says Not Delivered - you've tried to send an SMS (Green coloured message boxes) to a few people over a few days and it's still not working. You've tried Settings->General->Reset->Reset Networking Settings,  Settings->General->Reset->Reset All Settings, restarting your iPhone, pulling the SIM in/out and read Apple's help page  - all to no avail.

Then the problem may be down to an issue with the Short Message Service Centre (SMSC) setting on your phone - which is used when sending SMS messages. It can be checked/modified from the normal Phone app - type in the the following codes into the keypad and hitting the call button to see what the current SMSC setting is:
*#5005*7672#
You should then see a grey box fill the screen with a message:

Setting Interrogation Succeeded
Service Centre Address

Immediately below this text you'll see the number that your phone is using as your SMSC. You can search the Internet for your operator's SMSC and if it doesn't match the one you see above then you can set it to the correct one using the following general code - you'll need replace in the SMSCNUMBER with your operator's SMSC (to the get the plus(+) symbol you need to hold your finger down on the zero key):
**5005*7672*SMSCNUMBER#

 e.g.: For Talk Talk UK:
**5005*7672*+447785011324#
 e.g.: For Vodafone UK:
**5005*7672*+447785016005#

e.g.: For O2 Prepay UK:
**5005*7672*+447802092035#

After you hit the call button it will bring up another grey box saying:

Setting succeeded
Service Centre Address
No Address

This means it's done and SMS/texting should work now - Try it and see!  (If not then revisit my suggestions in the first paragraph).

For other phones (e.g. Android) the above should work, but you can also apparently bring up a special Phone information screen where you can check and modify your SMSC by typing this code on the Phone keypad:
*#*#4636#*#*

The SMSC is usually stored in the SIM card on your phone - for SIM card reader folk (see my other article on SIM readers): it's at EF SMSP (short message service parameters), the full path to the file is 7F10 / 6F42 or using AT commands: AT+CSCA?. 

As to why the SMSC might have changed - feel free to comment on that!