Wrong number sending with text message

When I upgraded to I phone 12 I took out a new Vodafone contract through Carphone Warehouse. I had previously been with Vodafone directly. Vodafone wouldnt allow me to keep my number so I had to buy a simcard from GifGaff mobile and port my old number to GiffGaff. I then had to port the number from Giffgaff back to the new Voda account. That has worked fine but the phone keeps sending the GiffGaff number as my number instead of the original Voda number. Hope that makes sense. How do I get rid of the old Giff Gaff number of my new I phone?