Modern day telephone systems need a method that allows multiple users to share the telephone systems at the same time. This can be achieved by using :

FDM (Frequency Division Multiplexing) – It utilizes the same mechanism as your television to divide the frequency available into multiple channels with each conversation being allotted a frequency. Once the available channels are exhausted, no more calls can be made, till another channel is freed. 

TDMA (Time Division Multiple Access) TDMA systems utilize time slots to transmit calls rapidly to their destination. Each call in a channel is first digitized and then placed on a Radio Frequency (RF) channel with other calls. There can be 3 to 8 time slots depending on the system. 

CDMA (Code Division Multiple Access) ñ The fastest and most efficient of all multiple call systems, a CDMA network will divide your digitized call into a number of packets, each with a unique code. The receiver will only receive those coded packets which are meant for it, out of the billions present, similar to data transfer over the internet.