My iMessages appear to default to be sent from a number that is not my number - when I go into settings it doesn't allow me to delete this number. To do it online it suggests sending a text to the number to enter but I don't have that number?

How do I get rid of a number where it shows that my imessages are sent from when I don't have that number - the de-register instructions say enter the number and text will be sent - as I don't have the number I am not getting the text to deactivate the number?

My settings won't let me delete the number as it is greyed out

I think when I got my new phone this would have been the number with the phone but at the same time I switched back to my old number so I have never used this number and don't know that for sure