Using __call to preprocess method calls to a class. How do I apply this for nested method calls?

I'm using the php __call on a wrapper class for several classes.

class Wrapper {
  public function __construct($obj) {...}

  public function __call($methodName, $params) {
    $this->preprocessSomeStuff();

    // call the original object's method
    $sourceMethod = new ReflectionMethod($this->obj, $methodName);
    return $sourceMethod->invokeArgs($this->obj, $params);
  }

This works great for the initial call, but doesn't work if the $obj has a nested method call, e.g.

class Thing {
  public function doStuff() {
    $this->doMoreStuff();
  }  

  public function doMoreStuff() {...}
}

With this object, calling:

$wrappedThing = new Wrapper(new Thing());
// preprocessing applied for doStuff(), not applied for doMoreStuff()
$wrappedThing->doStuff();