Most analog telephone systems in use in modern countries use multifrequency (MF) pulsing. This type of signaling consists of combinations of frequencies to send information other than voice over trunks. The combinations of two frequencies represents a pulse where each pulsed combination represents a digit. These frequencies fall within the speech bandwidth, so they can be sent over regular voice channels. In the USA phone subscribers use a form of MF pulsing called DTMF.