If you are going to use it to contact someone immediately, you can use the 'Meet Now' function in Skype for Business.
Click on the the calendar tab in Skype for Business.
Then click the Meet now button,
Please note - on a Mac you need to go to Conversations > Meet Now.
when you see the box below, Use Skype for Business (Full audio and video experience) should be selected, click ok
this will open up a meeting window with just you in it and then add your participant using the + icon in the top right corner of the meeting window.
You can then search for a participant, you can either search for a name to call someone on Skype for Business or put in a phone number to allow them to join via thier phone. Select the person or number when it comes up and click ok. You can up to 50 participants.
Once they answer they will be added to the meeting.
Arranging meetings via the calendar or using Meet now connect in a different way than contacting someone through the contact list and can have a more stable connection.