Since there is a bit of confusion when it comes to how dial in numbers work with Skype for Business I figured our partners might want a bit of extra info. Adding dial-in numbers to Skype for Business (the artist formerly known as Lync) involves buying a package of numbers and minutes from PGi or InterCall, who both have certified voice service infrastructure built into O365 data centers. Once your client has a number assigned, the O365 escalations team here at AppRiver can show you how to have it automatically added to their Skype for Business meeting invitations. The MSFT support article that covers this can be found here.
Per this article from last week, native Skype for Business PSTN Conferencing is in preview now in the US and 14 other countries. Microsoft is also previewing Skype for Business PSTN Calling with number portability in the US now. More to come on that as it develops.