ITU-T RECOMMENDATION E.180 – Technical characteristics of tones for the telephone service
1 General
2 Electrical levels for tones
3 Acoustical levels for tones
4 Dial tone
5 Ringing tone
6 Busy tone and congestion tone
7 Special information tone
8 Warning tone to indicate that a conversation is being recorded
9 Payphone recognition tone
10 Call waiting tone
11 Caller waiting tone
12 Machine recognition of tones
ANNEX A – Digital generation of tones
ANNEX B – Examples for limitation of spurious components of the dial tone with respect to interference with the frequencies recommended for pushbutton telephone sets in Recommendation Q.23