Using a traditional telephone line to accept telephone calls for on-air use presents an interesting dilemma: Audio from a phone line contains the voices of both parties.
If one were to simply mic their telephone speaker, the resulting audio would contain the Host Audio as well as the Caller Audio. Feeding this signal into the mix would be problematic. The resulting Broadcast Audio would contain broadcast-quality Host Audio mixed in with telephone-quality Host Audio. A situation like this would certainly lack the professional sound that most Streaming Broadcasters seek. On top of the unprofessional sound, this solution offers no way of feeding Host Audio back to the caller.
In order to properly air Caller Audio, the caller signal must be separated from the Host Audio. The Caller Audio is isolated from the telephone connection, and fed into the Broadcast Audio mix. The Broadcast Audio mix must then be sent to the caller in a format know as MixMinus, which means the Caller Audio is removed from the Broadcast Audio mix fed to the caller as the Host Audio. Failing to remove the caller audio from the Host Audio will cause the caller to receive his or her own audio back, which will create a slight and irritating delay.