  We have carried out $^{125}$Te nuclear magnetic resonance (NMR) in a wide
temperature range of 1.5 -- 300 K to investigate electronic properties of
Ge$_{50}$Te$_{50}$, Ag$_{2}$Ge$_{48}$Te$_{50}$ and Sb$_{2}$Ge$_{48}$Te$_{50}$
from a microscopic point of view. From the temperature dependence of NMR shift
($K$) and nuclear spin lattice relaxation rate (1/$T_1$), we found that two
bands contribute to the physical properties of the materials. One band overlaps
the Fermi level providing the metallic state where no strong electron
correlations are revealed by Korringa analysis. The other band is separated
from the Fermi level by an energy gap of $E_{\rm g}/k_{\rm B}$ $\sim$ 67 K,
which gives rise to the semiconductor-like properties. First principle
calculation revealed that the metallic band originates from the Ge vacancy
while the semiconductor-like band may be related to the fine structure of the
density of states near the Fermi level. Low temperature $^{125}$Te NMR data for
the materials studied here clearly show that the Ag substitution increases hole
concentration while Sb substitution decreases it.
