  We report high pressure Raman experiments of Black phosphorus up to 24 GPa.
The line widths of first order Raman modes A$^1_g$, B$_{2g}$ and A$^2_g$ of the
orthorhombic phase show a minimum at 1.1 GPa. Our first-principles density
functional analysis reveals that this is associated with the anomalies in
electron-phonon coupling at the semiconductor to topological insulator
transition through inversion of valence and conduction bands marking a change
from trivial to nontrivial electronic topology. The frequencies of B$_{2g}$ and
A$^2_g$ modes become anomalous in the rhombohedral phase at 7.4 GPa, and new
modes appearing in the rhombohedral phase show anomalous softening with
pressure. This is shown to originate from unusual structural evolution of black
phosphorous with pressure, based on first-principles theoretical analysis.
