A system and method for providing a call back option to a customer of a call center. The content-sensitive identifiers enable a caching proxy or a caching server to unambiguously determine the version or contents of media data cached by the caching proxy for a particular data pointer or data reference such that an appropriate version of the media data can be served to a requesting client system in an efficient and economical manner. A current cancel circuit has a first monitor transistor for flowing a third off-leak current that is smaller than the first off-leak current, and a cancellation circuit for flowing the first off-leak current to the low power potential responsive to the third off-leak current.