You can also make calls to phone numbers via your Skype for Business.
If the person is already in your contact list simply hover your cursor over the profile photo and click the receiver icon.
If the person is not in your contact list you can simply dial in the number and place the call that way.
You do that by clicking the keypad icon in the top menu. Then simply use your mouse to dial the number and click the Call button.
You can also key in the number directly in the search field and press Enter on your computer's keyboard.