Comment by ColinWright
2 years ago
We still have a land line. When a call comes through the phone often gives a gentle "peep", then a pause, then goes full-on ring. I've started to react to the "peep".
But every evening, mostly around 21:00 or so, the phone gives a gentle "peep" without then ringing.
I wonder if it's a line test?
Crap electronic ringer, probably. If you put a scope on the line, you should be able to see what's happening. Remember to be prepared for higher voltages, up to 400V.
There are various weird, obsolete signals in analog phones. Ring pulse alerting signal. ALIT test. Polarity reversal. Ring to ground. Ground start. Caller ID (1200 baud FSK between the first and second rings) DSL. Basic talk and ring was standardized around 1900, and everything else is backwards compatible. Ringers are supposed to ignore all that stuff. People who implement Asterisk PBXs are into this.
Here are some actual waveforms, if anybody cares.[1]
[1] http://www.adventinstruments.com/Products/AI-5120/Screenshot...