Why doesn't __call__ lead to infinite recursion?

[Patrick Lioi]

def foo(): pass

foo is a function
foo is a callable object
foo has method __call__ defined

foo.__call__ is a function
foo.__call__ is a callable object
foo.__call__ has method __call__ defined

foo.__call__.__call__ is a function...


This seems to go on forever.  How does calling foo() not lead to 
an infinite loop while trying to execute?