That clearly is a plugin field of “Advanced Custom Fields”.
The function:
the_field();
is one of the utility functions of the plugin. The purpose of “Advanced Custom Fields” is to enable users with less coding-knowledge to add custom fields to a theme/template.
So, in order to reverse engineer the whole thing, you propably need to install and activate “ACF” (very often used as an abbreviation for Advanced Custom Fields).
Activate it, and see what happens. In case your installation is missing the necessary data for the plugin, you might have to add them via ACF in the backend.
Try to find the field in the backend, which says “phone_number” it should be there, if it is not existent – create it.
Here is the plugin’s website and here the documentation of the field.