On August 14, 2010 a new area code was introduced to overlay existing area code 715 covering norhern Wisconin towns and cities such as Ashland, Eau Claire or Marinette. The new code is 534 and it will cover the exact same geographic area served by area code 715. The mandatory 10 digit dialing period for both codes was implemented on July 17, 2010.
See Nanpa announcement on the introduction of area code 534 (in PDF format)