Skype for Salesforce is a free app from the AppExchange that allows you to connect with your internal Salesforce users via Skype chat, audio calls or video calls. Your organisation needs to be licensed for Skype for Business online to use this app. Streamline collaboration with the latest breakthrough integration between Salesforce and Microsoft.
Users can chat using Skype for Salesforce on any supported browsers for Lightning Experience. Users can make audio and video calls using Skype for Salesforce from Microsoft Edge or Apple Safari® only.
Documentation for the app is here: Skype for Salesforce Overview