Properties

An interface can define properties that must be implemented for classes. Implementator class must define a property with the same signature and access permissions to the property's get and set.

As any GObject property, you can define a body to property's set and get in the implementator class, when no body is used values are set and get by default. If given, you must define a private field to store the properties values to be used outside or inside the class.

Callable interface definition, defines an answering property. In this case this interface defines a answering with a protected set, allowing a read only property for any object using an instance of Callable, but allows class implementors to write values to it, like TechPhone class does when implements hang() method.