When I iMessage myself, the text originates from my Google Voice number

My phone is set to start new conversations only from my iPhone number. I have deleted my Google voice number from my iPhone and erased all messages using my Google voice number from it. It only happens when I text myself. Weirdly, my iPad and my iMac don't have this problem so it has to be a something to do with my iPhone. I have spoken at length to Apple Help and they can't figure it out as it should be impossible that this is happening.