International call progress tones for IP-PBX and VoIP telephony
Telephone Line Signalling
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- Tone dialing uses DTMF tones. The acronym DTMF stands for dual-tone multi-frequency.
- Duration is typically 100 ms with an interruption of 100 ms.
- DTMF signals consists of a pair of tones, one from the low group (697,770,852,941), and one from the high group (1209,1336,1477,1633), yielding 16 combinations.
- Nominal levels are -9 dBm for the higher frequency tone and -11 dBm for the lower frequency tone. The difference in level is known as 'twist'.