Q: how can I select my phone number on iMessages on a mac

Hi. Follow the instructions on this page: iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage - Apple Support

Look for iMessage settings on an iPad and OS X using the same Apple ID.

Hope this will help

Doesn't work for me.  Here are the instructions you referenced - see my notes in red:

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Get started on your Mac

To link your phone number to iMessage or FaceTime:

1. Open the Messages or FaceTime app. - OK
2. Sign in with the same Apple ID that you use on your iPhone. - OK
3. Select the phone number and email addresses that you want to be reached at. - I was already signed in w/Apple ID so I signed out and back in.  When I signed back in (several attempts) I do not get an option to select a phone number.

After you sign in to iMessage or FaceTime on your Mac, your iPhone will get a notification saying that your Apple ID and iPhone are now being used with iMessage or FaceTime on your Mac. You’ll get a notification for every email address or phone number that you enable on each additional device. - I did receive this notification on my iPhone.

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Any other suggestions?