Why does my PC still use Skype for Business or another softphone to initiate calls?

You need to change a few settings on your computer.

Windows: 

Press the Windows button and search for Default apps in Windows. 
Scroll to the bottom and pick Choose default apps by protocol. 
Find Tel protocol. 
Press the application shown here.
Pick Microsoft Teams instead. 

Mac:

Open Facetime. 
Go to Facetime Preferences. 
Choose Default for calls to be Microsoft Teams.