  The model of NH$_3$CH$_2$COOH$\cdot$H$_2$PO$_3$, modified by taking into
account the piezoelectric coupling between the ordering structure elements and
the strains $\varepsilon_i$, $\varepsilon_j$, is used for investigation of the
effects that appear under external pressures. Within two-particle cluster
approximation, the components of polarization vector and static dielectric
permittivity tensor of the mechanically clamped and free crystals, their
piezoelectric and thermal characteristics are calculated. The effect of
hydrostatic pressure on the phase transition and the calculated physical
characteristics of the crystal is studied. A good quantitative description of
experimental data for these crystals is obtained.
