Many websites use the tel: protocol to allow seamless "Click to Call" functionality with Teams. To ensure you have Teams 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 Microsoft Teams 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 Teams and call us!