In the history of smartphones, never before had such a tone been heard at such a volume. In Android, specifically on Google-made smartphones, there's a sound that's played at the end of any phone call ...
Recall dial tone-13 dBm0 3 bursts of 0.1 s followed by a continuous tone (note c) Recall dial tone Modern PABX only (note g)-16 dBm0 3 bursts of 0.1 s followed by a continuous tone (note c) Busy ...
Telephone dial tones are one aspect of modern life that we take for granted, but there is a history there. Wikipedia offers some interesting reading on the subject. Back in 1954 when Bell Telephone, ...