We investigate the relation between sudden phase anomaly (SPA) amplitudes (Δ$varphi$) and solar X-ray flares importance and study if it has a solar activity cycle dependence. We find a very significant correlation between Δ$varphi$ and the X-ray fluences, FX (time-integrated photon fluxes) in the range 0.5--2 ¿. Compared with earlier works, the improvements of the Δ$varphi$ versus FX relation allow us to study separately solar events as a function of their occurrence in the solar activity cycle, and we find that the Δ$varphi$ versus FX relation is different depending on the epoch within the solar cycle. In particular, a minimum X-ray fluence of 2.5 ¿ 10-6 J m-2 is needed during solar activity minimum to trigger a SPA, while during solar maximum, 7.0 ¿ 10-6 J m-2 is required. Similarly, a solar flare will produce a SPA during solar minimum that is greater by ~2.6¿ M m-1 than would a flare of the same size during solar maximum. These results confirm recent findings about the dependence with the solar activity of the ionospheric undisturbed D region sensitivity. A possible consequence would be the monitoring of the long-term solar irradiance, which maintains the D region, through measurements of VLF wave propagation properties.