From:
http://docs.python.org/reference/datamodel.html#the-standard-type-hierarchy

"Class instances
    Class instances are described below. Class instances are callable
only when the class has a __call__() method; x(arguments) is a shorthand
for x.__call__(arguments)."

The following program demonstrates otherwise regarding that last statement. 

def call(self):
    print "inserted __call__ in object of class A"

class A(object):
    def __call__(self):
        print "__call__ method in class A"
        
x = A()               # Equates: x = type(A).__call__(A)
x.__call__ = call

x()                   # Calls the method of class A.
x.__call__(x)         # Calls function "call".
type(x).__call__(x)   # The correct longhand of x() IMHO


If I were to rephrase the documentation:
"Class instances
    Class instances are described below. Class instances are callable
only when the class has a __call__() method; x(arguments) is a shorthand
for type(x).__call__(x, arguments)."