Imessage uses my pre-ported number
A while ago I ported my number from my network onto a new sim card. I've just found out that whenever I send a message with iMessage it seems to be coming from the old number.
My Apple ID account shows both the old and the correct (ported) number, but the old one is the one at the top of the list which I'm prevented from deleting.
From searching for solutions i've tried a few things, including:
Switching off iMessage and Facetime, turning phone off for 10 mins then switching them on again.
Changing my AppleID password.
Resetting my network settings on the phone.
None of these worked.
Any ideas?