Office 365 Enterprise E5

The Office 365 Enterprise E5 plan includes most Skype for Business features. There are just a few additional options.

To add this feature
Here's what you need to buy
Audio Conferencing is included!
Once you buy the license, Microsoft will be your audio conferencing provider.
Toll-free numbers for dial-in access to your meetings, and the ability to dial out from a meeting to add someone by calling any telephone number in the world.
Communications Credits
Phone System and voicemail is included!
Calling Plans: buy a Calling Plan from Office 365.
Or, use the telephone service from your existing provider.