Dear Miaaahhh
Thank you for posting in the Microsoft community.
Some of the new style numbers (5G numbers and virtual numbers) may not be available in Microsoft accounts for the time being.
If it is convenient, you can test it with other number segments from other carriers.
In addition, the user environment will be detected when adding a cell phone number, so we recommend you to turn off the VPN proxy and use the privacy mode of your browser.
Of course, you can use your mobile browser to add a cell phone number to your Microsoft account.
Last but not least, Microsoft is updating the list of carrier numbers, this process may take some time, please wait patiently.
You can log in
account.microsoft.com
Click Feedback at the bottom right corner - submit the cell phone number you can't add and a screenshot of the error report.
Best Wish
Shawn.Z-MSFT | Microsoft Community Support Specialist