In Skype for Business you can make phone calls just by typing a number. In the search box, type the number and press enter.
A call dialog box should appear:
To hang up the call, press the red hang up button towards the bottom of the call dialogue box.
Calling a Skype for Business contact
To make a call to a Skype for Business contact, right click on them, hover over Call and click Skype for Business call. This will bring up a call dialogue box like before and make the call direct to the users Skype for Business client.
NOTE: Remember, calling the DDI of an internal co-worker will result in a call charge, however, starting a native Skype for Business call will not. Always select the Skype call or Lync call option, instead of the Work option.