Lattice dynamics simulation of TlH(2)PO(4) and TlD(2)PO(4) crystals was performed within the semi-phenomenological atomistic model in the high temperature P2(1)/b2/c2(1)/n structural phase. Combining the results of phenomenological Landau-Ginzburg theory and lattice dynamics simulation, it was shown that the ferroelastic phase transition in TlH(2)PO(4) occurs due to the bilinear interaction between the soft B(3g) optic and B(1u) acoustic modes. According to our simulation, the hydrogen H(1) atoms placed on the shorter hydrogen bonds play the key role in the ferroelastic phase transition in TlH(2)PO(4), whereas the D(2) atoms located on the longer hydrogen bonds exert the main effect on the antiferroelectric phase transition in TlD(2)PO(4) crystal.