  A modified RSJ-model for the coupling of intrinsic Josephson oscillations and
c-axis phonons in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+\delta}
and Bi_2Sr_2CaCu_2O_{8+\delta} is deveoped. This provides a very good
explanation for recently reported subgap structures in the I-V-characteristic
of the c-axis transport. It turns out that the voltages of these structures
coincide with the eigenfrequencies of longitudinal optical phonons, providing a
new measurement technique for this quantity. The significantly enhanced
microwave emission at the subgap structures in both the GHz and THz region is
discussed.
