Factory reset iphone with new apple id - phone not ringing and phone calls going to old voicemail

We are currently transitioning between the US and UK. We got an EE carrier UK sim and put it in my partners old iphone 6. It worked fine.

We bought an iphone SE 2020 for the EE carrier UK sim and set it up with his apple ID. This phone would not ring if you dialed the UK number and you got transferred to a US voicemail message instead of the EE UK voicemail system. It also showed all my partner’s recent US calls and texts as its recent calls and texts, even though it was a separate number/device.

I called apple support and they got me to change settings so the two devices were no longer sharing texts but it still continued to show the US phone’s recent calls in its recent call list even after being switched off. It was still not ringing when the number was dialed and still got transferred to a US voicemail system.

We factory reset the iphone for the EE UK sim and set it up with a completely separate, new apple id thinking this would solve the problem. If the phone is switched on it receives texts, but it still does not ring if you dial the number and still gets transferred to an American voicemail message instead of EE’s UK voicemail system. If you call when the phone is switched off, you get transferred to the EE UK voicemail system.

Can anyone help. We did not have this problem when the sim was in the iphone 6.