Many websites use the tel: protocol to allow seamless "Click to Call" functionality with Skype for Business. To ensure you have Skype for Business set as your default telephony application please follow these steps (Win10):
1) Go Windows Settings > Apps > Default Apps
2) Scroll to the bottom and select Choose default apps by protocol
3) Scroll down to TEL protocol and make sure Skype for Business is set as the default application.
4) To test this has been set up, please go to https://via.co.uk/ and click our number at the top - this should launch Skype for Business and call us!