I use Skype for my calls and so download and install the application on my PCs. However on Windows 10 it pre-installs Skype Preview which I found takes over the normal Skype application and therefore doesn't allow me to send/receive calls. Simplest solution I found was to uninstall Skype Preview using Powershell.
Open PowerShell as administrator and run:-
Get-AppxPackage *skypeapp* | Remove-AppxPackage