Human speech contains frequency components roughly between 100Hz and 8kHz, but for commercial telephony, only the most important frequencies for intelligibility are transmitted. The telephone system limits the transmitted audio bandwidth to approximately 300Hz−3400Hz, though sometimes it's cited as 300Hz−3100Hz, depending on standards and equipment. This limited range saves bandwidth while still allowing speech to sound natural and be easily understood. So, the frequency range adequate for commercial telephonic communication is approximately 300Hz to 3100Hz( or 3400Hz).