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Appendix
Acoustic signalsInternal dial tone
3 x 0.2s tone and 0.3s pause, sequence repeated after 0.5s pause.
External dial tone
Steady tone
Ringing tone
Tone Pause Tone Pause Tone
0.8s 2s
Busy tone
0.5s 0.5s
Acknowledgement tone
Tone
0.8s
Error tone/range tone
4x0.1s