iMessage and FaceTime number activation fails after moving to Vodafone—restore fixed it for only two hours

I’m hoping someone can help with a persistent problem on my iPhone 16 Pro Max.

Since moving my existing mobile number from Sky Mobile to Vodafone, I’ve been unable to keep my number activated for iMessage and FaceTime. Normal calls and mobile data work.

I’ve spent considerable time troubleshooting with both Apple and Vodafone. Apple suggested Vodafone might be blocking international activation SMS messages; Vodafone says there is no such restriction. Neither has resolved the problem.

Today I tried the following:

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• Backed up the iPhone to iCloud.
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• Erased it and temporarily set it up as a new phone, without restoring apps or settings. My number still showed the activation spinner.
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• Erased it again and restored my original iCloud backup.
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After restoring the backup, iMessage worked using my mobile number—not just my email address—for approximately two hours. It then stopped working again.

The current position is:

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• My number appears under Messages → Send & Receive, but I cannot get a tick beside it.
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• There is currently no activation spinner beside the number.
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• My Apple Account email address can be selected and works.
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• My Vodafone eSIM is enabled, and the correct mobile number appears under Mobile Service in +44 format.
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• I can see only one mobile line.
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Has anyone experienced their number successfully activating and then becoming unavailable again a couple of hours later, particularly after transferring a number to Vodafone?

I would especially appreciate suggestions on what Apple or Vodafone should investigate next. I’ve already tried the usual troubleshooting repeatedly and would like to avoid another full erase unless there is a specific reason for it.

Thank you.