Cannot remove old number from Two-Factor Authentication

I have recently changed my mobile number because I have moved to the UK from New Zealand. I have successfully added my UK number as a trusted number in settings and it is the only number showing on the sign-in and security page in settings. But if I tap on two-factor authentication, that page shows my mobile device (an iphone 14 pro) and if I tap on the device info it still shows my old NZ number as the primary number, with no option to edit or remove that number. My UK number shows as the travel number. How do I remove the NZ number? I have tried the official website - same problem.