Why are traditional telephone lines limited to about 3-4 kHz bandwidth? According to Wikipedia the frequency range of the plain old telephone service is 300Hz to 3.4kHz.
What is the reason for this? Why can't the cables and equipment tolerate higher frequencies?  Is it something to do with the material used in the cables, or the thickness of the cables?
Why would the frequency of the signal, which as I understand it is the number of electrical pulses per second, be limited in this way?
 A: In the old days of analog electronic data transfer, high bandwidth cost more in terms of the technology required, so all data transmission technologies were costed out for bandwidth requirements and no more bandwidth was bought than absolutely necessary for any particular application.
In particular: high frequency analog data transmission was strongly limited by things like the parasitic capacitance of the wiring and switchgear technology; the remedies were expensive and limited in effectiveness.
What to do? Well, for the purposes of reliable speech recognition, experimentation determined that satisfactory results could be gotten even when the frequency response of the telephony system was limited down to the range of just 300Hz to 3000Hz. This bandwidth range or something close to it then became the default for both analog telephony and long-distance radio transmission, a state of affairs which has persisted for almost 100 years- right through the first few iterations of cellular phone technology.
Now that the technical limitations of analog data transmission are moot, digital sound processing in cell phones is routine and the high-frequency response of digitally-based voice transmissions systems is not costly anymore.
A: Telephone company operators used to physically patch one wire to another to make a dedicated line from end user to end user. Certainly parasitic capacitance had a limiting effect.
But today, packetizing voice to data packets over digital lines allows many conversations to travel over fewer copper wires and this saves lots of money.
The industry settled on sampling each subscriber line at 8khz where the signal can take any value of 8 bits. And with sampling, the highest frequency  that can be sampled is about 1/2 the sampling rate.  That means 8khz sampling can at best sample a 4khz signal which is sufficent for vocal over telephone.
