  We study capability of $f(R)$ gravity models to allow crossing the phantom
boundary in both Jordan and Einstein conformal frames. In Einstein frame, these
models are equivalent to Einstein gravity together with a scalar field
minimally coupled to gravity. This scalar degree of freedom appears as a
quintessence field with a coupling with the matter sector. We investigate
evolution of the equation of sate parameter for some cosmologically viable
$f(R)$ gravity models in both conformal frames. This investigation (beyond mere
theoretical arguments) acts as an operational tool to distinguish physical
status of the two conformal frames. It shows that the two conformal frames have
not the same physical status.
