Measurements are reported of the temperature dependence of the resistivity, Hall effect and cyclotron resonance, in n-type InP samples which show activated conduction. After an important correction for Fermi level variations was made, accurate values of threshold energies are deduced. It is shown that the threshold for DC conduction is higher than for both the Hall effect and the cyclotron resonance effect. An explanation is given in terms of the long-range impurity potential fluctuations, and is based both on an effective medium model, and on the manner in which carrier localisation explains the Hall and resistivity results in 2D systems.