On this page you can learn how you can call a contact using Skype for Business Android app.
To search for someone in the company directory, tap Search in the company directory, and then start typing a name, phone number, or email address in the search box. The list of contacts will start to display as soon as you begin typing, and the results will be refined as you continue to type the name you are searching for.
To view or edit your Skype for Business contacts, from the main screen, tap Contacts to view your Favorites, Other contacts, and Added by contact lists.
Depending on how you set your VoIP call options, when you make a call it could use Wi-Fi or your cellular data plan. For more information, see Skype for Business, Manage Settings for Android App