A THz-probe spectroscopy scheme with laser-induced tunable single cycle pulses of coherent synchrotron radiation as emitted from a laser-energy modulated relativistic electron bunch at the BESSY II storage ring is devised and adapted to reveal the dynamic THz transmittance response [1] in epitaxially grown phase change materials upon 800 nm fs-laser excitation. Amorphous (a-) and crystalline (c-) of the prototypical Ge2Sb2Te5 (GST) alloy are probed with single cycle THz pulses tuned to the spectral range of highest absorption contrast at 2 THz. After an initial instantaneous sub-picosecond (ps) dynamic THz transmittance drop, the response of a-GST in that range is dominated only by a short recovery time of 2 ps of the excited carriers. On the contrary, the behavior of the c-GST response displays a short decay time of 0.85 ps followed by a slow one of 90 ps, suggesting that vacancy layers in ordered c-GST …