Hello and welcome to Apple Support Communities, corpho!
I understand you're trying to add your phone number to Messages on your MacBook Pro. I'd be happy to provide information to help you troubleshoot this issue.
From the article you linked, sign out of both iMessage and FaceTime on your iPhone and MacBook Pro: Add or remove your phone number in Messages or FaceTime
"Remove your phone number
If you don't want calls or messages for your iPhone on your iPad, iPod touch, or Mac, you can remove your phone number. There are two ways to do this:
- On your iPhone, go to Settings > Messages > Send & Receive. Select your Apple ID and tap Sign Out. Then go to Settings > FaceTime, select your Apple ID and tap Sign Out.
- If the phone number that you want to remove belongs to an iPhone that you can't access, change your Apple ID password. This removes all phone numbers from your iPad, iPod touch, and Mac. Calls or messages for the removed numbers won't appear on those devices anymore.
If you have questions, contact Apple Support."
"Turn your phone number on or off for your Mac
After you set up your phone number, you can turn it on or off for Messages and FaceTime on your Mac.
- Open Messages or FaceTime. If asked, sign in with the Apple ID that you're using with Messages and FaceTime on your iPhone.
- For Messages, choose Messages > Preferences, then click Accounts and select your iMessage account. For FaceTime, choose FaceTime > Preferences, then click Settings.
- Turn your phone number on or off:
Once you've signed out of both iMessage and FaceTime on both devices, sign back in first on your iPhone. Then, proceed to sign in on your MacBook Pro.
Thank you for using Apple Support Communities.
Enjoy your day!