iMessage Activation failed on iPhone

I have been helping a friend with her replacement second-hand iPhone SE 3rd gen. We have transferred everything from her old iPhone and it is all working except iMessage and FaceTime, which cannot be turned on - Activation fails. The same phone number and email address were used on the old phone and iMessage and FaceTime worked normally. She can receive SMS messages on Messages with this phone number.

It is connected to both wi-fi and a 4G network

It is on iOS 18.7.1

The time, date and time zone are correct

iMessages and FaceTime are both turned off in Settings>Apps and cannot be turned on - we get the Activation Failed message. Her phone number is shown with a tick in the Send & Receive list but not her email address and it is not possible to untick or add either.

Her correct phone number and email address are shown in Apple Account Sign-In & Security. We have signed out of Apple Account and signed in again.

We have also shut down and restarted the phone a few times and reset the Network Settings twice.

Any suggestions, please?