The 1040 has a spot for both a phone number and an email address. The 1040 instructions make it completely clear that both are optional.

  You have the option of entering your
  phone number and email address in the
  spaces provided. There will be no effect
  on the processing of your return if you
  choose not to enter this information.
  Note that the IRS initiates most contacts
  through regular mail delivered by the
  United States Postal Service.