Did you read Zig's post today – https://blogs.office.com/2015/07/01/announcing-preview-of-new-skype-for-business-services-in-office-365/
I am one of the employees already using the PSTN Conferencing and Cloud PBX with PSTN Calling features.
Being one of the earliest users I have uncovered a few bugs but with it working, you can't describe the difference.
The meeting invite has a new Bridge number and Conference ID, but it still works by joining and calling.
Other than getting a new phone # from Portland OR and confusing the wife on the first call, it just works.
Having started with LCS 2003 and moving to our Office 365 Dedicated team with Lync 2010 to do Voice, where we are going is amazing.
As I work with our corporate deployment I will seek to share insight into the complexities that customers will need to prepare for such as how do you handle number porting.