How did this happen?
Well, I can't send SMS text messages with my new phone because it doesn't have a SIM card, it's still in my old phone, but I did try it in my new phone so the google messages app is set up with my number
My old phone has a different SMS app
So, I send an SMS text from my old phone to my partner, and when my partner replies using google messages app it sees that I'm available in chat and so sends the reply as a chat message which is received by my new phone