The latest type of high speed modem is built per a standard called V.34. The data speed that can be supported on an analog circuit is a function of the bandwidth of the channel and the noise on that channel. Wide bandwidth, low noise connections, can carry more data than narrow bandwidth, high noise connections. The telephone network was designed to provide a connection with enough bandwidth so that voices could be heard clearly and that the speakers voice could be recognized. That's why the bandwidth of a voice channel is from 300 Hz to over 3000 Hz.
Noise is inherent in a telephone system and can have many sources.
The telephone company will try to control noise so that it is not objectionable to most people. Any attempt to increase bandwidth or reduce noise, costs money. So the bottom line is that the telephone company will try to provide a good quality voice connection at a reasonable cost. The telephone network is designed to provide good voice service on most connections at a reasonable cost. Any connection can be made up of combinations of local facilities, multiple switch connections, and multiple interoffice facilities.