• In a group conversation, use the Start call button.
• If the person you want to make an audio call to has a Skype Number or landline associated with their account, you can use Skype Credit or your Skype subscription to contact that number.
• Note: The call audio is treated to reduce the visibility of missing audio bits, filter out echo, and maintain audio quality (up to 120 milliseconds).
• To hang up, select the end call button at the end of a call.
• Tip: You can instantly transform any discussion into a phone call by selecting the audio or video call button in a one-on-one chat or the Start call button in a group chat.
• Note: If your mobile address book and Skype contact list are linked, you can make Skype calls directly from your device's contact list. To call someone, simply tap the Skype button next to their name.

Read More: How does Skype video call work?