Thank you for your post!
I understand that you're having issues adding your Cell Phone as an Authentication method within your user's Azure AD Profile.
When it comes to adding other authentication contact information to your actual signed in user's Azure AD Profile, you'll have to use the Access Panel Profile, specifically the My Sign-Ins Security Info page.
If you're trying to add Authentication Methods for other user's within your Azure AD tenant, the UI experience will allow you to add it via the Portal.
If you'd like the UI experience to be different, I'd recommend leveraging our User Voice forum and creating a feature request, so our engineering team can look into implementing this.
I hope this helps!
If you have any other questions, please let me know. Thank you for your time and patience throughout this issue.
If the information helped address your question, please Accept the answer. This will help us and also improve searchability for others in the community who might be researching similar information.