Including Employer Phone Numbers Is Not Common
Although resumes should provide all relevant information related to previous employment, including the phone numbers of past employers is not standard practice when writing resumes. When employers have stacks of resumes to review, seldom will they take the time to look contact a previous employer who is not on your list of references. Only your references should have full contact information.
There Is No Harm in Including Phone Numbers
While including phone numbers might not be a common practice, there is really no harm in listing phone numbers under your work history information, and it will have no substantial adverse effect on your resume itself. If your potential employer wants to contact a former boss who is not on your list of references, then including the numbers will actually make your resume more user friendly and easier for the reader to navigate.
Including the phone numbers of previous employers in the resume proper is not common since you will include such information on a formal list of references; however, including the numbers is okay and will make it possible for a reader to contact references even before formally requesting your reference list.