Add new field to node form on validate
I'm guessing what you actually want (from a user story perspective) is if a certain condition is met another, otherwise hidden, field is shown. As @geoff points out that field would need to exist on the content type.

I believe the correct way to do this within the Form API is with the #states parameter in order to conditionally show/hide the phone field. 
https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7#states