As the meeting organizer, you can dial out using a Skype for Business client to let other people join the same meeting using their phone. Although it's not required, when you are dialing out to someone, it’s recommended that you dial the full number, including the country/region code. To get dial-out to work:
You can dial-out only if you join a meeting using a Skype for Business client.
You as the meeting organizer have been enabled for dial-in conferencing.
Step 1: In the invite meeting you can use the Invite More People option to dial out to a phone number.
Step 2: Enter the full phone number including the country/region code in the box.
Supported countries and regions
Dial-out is only available to some countries/regions. To see a complete list under PSTN Conferencing, Supported countries/regions for Skype for Business Online PSTN Services.