This python script will detect modem events like DTMF Digits, Busy Tone, Silence etc from phone line using Raspberry Pi 3. Seems to do audio signal analysis to do this.
A page that generates different kinds of waves for you to listen to. Defaults to 440 Hz (middle A). Has other kinds of tone generators on the side, too.
The Test Call is the homepage of a free (and legal!) service that offers some telephony debugging services, such as echoback (say something, it records it and plays it back), DTMF identification, a 4KHz sweep for audio quality, and caller ID checking.