The click-to-call (or click-to-dial) feature lets you dial phone numbers by clicking specially formatted links on websites, CRM or other web or native applications.
How does it work?
It works by associating the desktop client (currently supported on Windows only) as a default app for the tel: and callto: protocols in the operating system. After that, the system will launch the associated app when clicking on the tel: or callto: links in any web or native application that supports functionality.
On the web pages where phone numbers are not formatted as either tel: or callto: links, you can install a browser extension (e.g., Telephone Number Detection) that parses a web page and turn any phone number it could find on the page into the properly formatted links, enabling click-to-call functionality.
Enabling click-to-call
First, install the desktop app on your Windows device.
On the Start menu, select Settings > Apps > Default apps.
Scroll down and navigate to Choose defaults by link type (or Choose default apps by protocol in Windows 10).
In the list of protocols, associate the app 
to the CALLTO and TEL protocols.
The click-to-call functionality now must be enabled.