After announcing the Skype for Business preview (formerly Lync), Microsoft has announced today that PSTN support will be natively available in Office 365 (starting with US first this summer) 
See the announcement here http://blogs.office.com/2015/03/18/skype-for-business-is-here-and-this-is-only-the-beginning/ (section The right cloud for communication and productivity in the middle of the page)
You can also sign in to be part of the US preview here http://updates.skypeforbusiness.com/