Under the Admin -> Telephony & SMS page, you can set specific telephone links to be added to phone numbers in LegalServer. You have a choice of having TEL: or CALLTO:// appended to the phone numbers. These will integrate with the default applications on your computer as set by the Operating System. Just as links that start https:// will open in a browser, you can define which local program will open the phone numbers. In Windows 10, those settings are defined by the "Choose Default apps by protocol" system settings page.