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Appendix
Acoustic signals
Internal 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