Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling framework. It utilizes the voice-frequency band over telephone lines among portable devices and different communications devices and switching centers. DTMF was first evolved in the Bell System in the United States. This framework utilizing a telephone keypad gradually supplanted the utilization of rotary dial and has become the industry standard for landline and versatile assistance. Other multi-frequency frameworks are utilized for internal signaling within the telephone network.