Insertion loss and Envelope Delay
For a number of reasons, the telephone company deliberately limits the frequency response of an analaog (POTS) telelphone line to roughly 300 - 3400 Hz. The characteristics shown in your graphs represent the filters used to accomplish that, rather than any physical characteristic of the transmission line itself.

The bandwidth was chosen to be a good balance between intelligibility for both male and female voices on one hand, and the efficient use of cable bandwidth for interoffice connections on the other. Initially, analog multiplexing (SSB FDM) was used to put multiple calls on one coaxial cable. Later, the voice audio was digitized at a sample rate of 8000 Hz and multiplexed using TDM.