iPhone number not listed in iPad at iMessage
Why is my iPad no longer receiving messages sent to and from iPhone?
iPad Pro, iPadOS 14
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You must explicitly enable Text Message Forwarding for your iPad on your iPhone.
On your iPhone:
Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.
Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:
Settings > [Your Name / AppleID] > Messages - set to ON
More information:
Fault finding if needed:
If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
I hope you find this to helpful in resolving the issue.