Comment by Animats

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...