What is the definition of dial tone?

Dial tone is a common sound on the phone, that is, there is a continuous "beep" sound in the receiver after off-hook. This tone is a dial tone, indicating that the automatic exchange or the other party's call center system is ready to continue, allowing users to dial the tone category. After picking up the phone, there was a continuous tone of "om-"in the receiver, which was a dial tone. It means that the automatic exchange or the other party's call center system is ready to connect, allowing users to dial the called number by dialing the ring-back tone of (450±25)HZ. If you hear the intermittent tone of "beep-beep-"(ring 1s, disconnected for 4s), it is a ring back tone, indicating that the called phone is ringing and you can wait for the incoming call. If there is no answer after calling 10 times, it means that the other party is not answering the phone. Put away the handle and dial again later. Busy tone When the caller hears a short tone of "beep, beep, beep ……" during dialing or after dialing the called phone number (ringing for 0.35 seconds and disconnecting for 0.35 seconds), it is a busy tone, indicating that the line is full or the called phone is using a long-distance notification tone. When calling and called users make local calls, they hear "beep, beep, beep …". Space tone When the user hears intermittent signal tones with unequal intervals after dialing (repeated 3 times, 0. 1s ring, 0. 1s ring and 0.4s ring, 0.4s ring and 0.4s ring), this is the space tone, which means to inform the calling user that the called number is an empty number or a restricted number. (450±25)HZ Call Tone If the user hears a continuous signal tone, the loudness changes to five levels and gradually increases from a low level, it is a call tone. Inform users who have not hung up for a long time to hang up quickly. (950±25)HZ reference: /tech/CTI/signal.html