Skype for Business: Understand Your Voice Options
Integrated Audio: Voice over Internet Protocol (VoIP)
Integrated audio uses VoIP and your computer's microphone and speakers for the audio portion of a meeting. If you have a Skype for Business handset or headset—or a webcam with a built-in microphone and speakers—you will be able to use this feature.
There is no cost for connecting to the audio portion of a meeting or conversation using VoIP.
Microsoft Office 365 Audio Conferencing
If you are unable to use Integrated Audio, Microsoft Office 265 Audio Conferencing allows you and other meeting participants to join the audio portion of the meeting using a phone. If you have Microsoft Office 365 audio conferencing, the dial-in conferencing numbers appear below the "Try Skype Web App" heading in your meeting invitation.