When sending an SMS from an iPhone, you might see an error message, but the recipient still gets the message. This happens when the stored number on your iPhone doesn't match the network's provisioned number, often due to:
Porting your number from another provider.
Changing your phone number.
Symptoms for iPhone
Note: This issue affects SMS (green messages), not iMessages (blue messages).
You’ll see a red exclamation mark and "Not Delivered" notice next to the failed SMS.
The Receiving Phone/Handset
The device to which the SMS are being sent will not show any errors, and all of the SMS will have arrived at their destination, even if your phone reports them as failed.
Resolution
To fix this issue, you will need to complete the following steps:
Disable iMessage
Update your "My Number"
Re-Enable iMessage
What you need to do:
1. Disable iMessage
You will need to navigate to the correct settings area in your phone as follows:
Find the Settings app on your phone
Select the Messages option
Find the iMessage setting and toggle it to
OFF(it will be green whenON)
2. Update your My Number
You will need to navigate to the correct settings area in your phone as follows:
Find the Settings app on their phone
Select the Phone option
Find the My Number setting
4. Update the number by typing it in (this will be the number you have ported in, or the new number you were provided if it was changed) and then click Save
3. Re-Enable iMessage
You will need to navigate to the correct settings area in your phone as follows:
Find the Settings app on their phone
Select the Messages option
Find the iMessage setting and toggle it back to
ON(it will be green whenON)
Please retest SMS send and receive to a non-Apple number (or send a message with iMessage still turned off) and verify these are now sent/received successfully.
In some scenarios, you may need to de-register your mobile number for iMessage to completely turn-off. You may follow the instructions given here