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iMore Question
Since upgrading my phone, my phone number has disappeared from iMessage on my iPad. How do I re-add it?
Try Resetting Your Network Settings.
Settings > General > Reset > Reset Network Settings.
Settings > iMessage > Send & Receive > check phone number as well as the email address.
I just was doing some research and realized that since you now have an iPhone, your iPad will only show the email address since your iPhone is now registered as your main iMessage location (since it has a number and it registers your data). I'm guessing you used the same Apple ID?
Make sure the number is checked in the settings
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