The Telephone module allows you to create fields that contain telephone numbers. The module is in the core and is disabled by default.
The telephone numbers can be displayed in different formats: e.g. as clickable link (for mobil devices) or as plain text. You can add a field ‘telephone to a contenttype, to a user or a custom block.
You can set a default value if you want.
In 'Manage display' you can choose the display you want: plain text or a clickable link.