Problem with sending text messages

[Yikes]

All Text messages I send say “not delivered”. Most of the time the messages are going through but the recipients are sometimes receiving it from a number that was originally assigned to my phone when I bought it a year or two back. I changed numbers right from the start. This has only been an issue in the last month. How can I fix this as I’m never sure if my texts do actually send?

[MI5 · Accepted solution]

@Yikes 

See tips here Guide: Text Message Tips (not sent or received) 

If you have an iPhone, check Settings >Phone >My Number

If android, check RCS settings within messaging app.

[Sam2812]

Thank you so much for this! O2 couldn’t manage to solve this problem for me but you have 

[MI5]

You're welcome @Sam2812