When a 10-digit or 11-digit number appears in an email, the number can appear as a hyperlink so that you can call the number by using the default telephony provider configured on your operating system.
In order for 10-digit and 11-digit numbers to be identified as phone numbers, you must select a default telephony provider.
Click Tools > Options.
Double-click Environment, and then click the Default Actions tab.
In the Default telephony provider drop-down list, select one of the following options:
None: Displays phone numbers as regular text and not as hyperlinks.
Legacy Windows Dialer: Uses a Windows dialer program that is no longer commonly used and may require additional hardware.
Tel Protocol Handler: Uses the Tel protocol handler that is configured on your system, which is likely the system default.