  We show that it is possible to use a massless field in the vacuum to
communicate in such a way that the signal travels arbitrarily slower than the
speed of light and such that no energy is transmitted from the sender to the
receiver. Instead, the receiver has to supply a signal-dependent amount of work
to switch his detector on and off. Because of that, this kind of communication
without energy exchange may be called "Quantum Collect Calling". This type of
communication is related to Casimir-like interactions and it is made possible
by dimension ---and curvature--- dependent subtleties of Huygens' principle.
