Our Lead records utilize multiple custom phone fields, appearing in multiple sections.  The tab order has been configured from top to bottom for each section.  When users create a new record, or edit one in a way that makes a custom phone field appear for input, the tab order of fields will skip custom phone fields.  The custom phone fields will eventually appear in the tab order, but not until the end of the sequence.  If the record is saved, and the user clicks edit, the tab sequence works as expected.