  It is shown that the hidden conformal symmetry, namely $SO(2,2) \sim
SL(2,R)_L \times SL(2,R)_R$ symmetry, of the non-extremal dyonic
Reissner-Nordstr\"om black hole can be probed by a charged massless scalar
field at low frequencies. The existence of such hidden conformal symmetry
suggests that the field theory holographically dual to the 4D
Reissner-Nordstr\"om black hole indeed should be a 2D CFT. Although the
associated AdS$_3$ structure does not explicitly appear in the near horizon
geometry, the primary parameters of the dual CFT$_2$ can be exactly obtained
without the necessity of embedding the 4D Reissner-Nordstr\"om black hole into
5D spacetime. The duality is further supported by comparing the absorption
cross sections and real-time correlators obtained from both the CFT and the
gravity sides.
