Count the number of times the phone rings before its answered
If you can identify one phone ring, then you can extract it and use it to construct a matched filter. The beginning of the wiki article is math heavy, but check out the example section. Additionally, Matlab has a page describing the matched filter from one of their toolboxes. You should really give it a try; it works like magic.

In the time domain, the matched filter is a cross-correlation of the signal against a known pattern (which is in this case a recording of the ringing). As a cross correlation it can efficiently be implemented using a DFT. The output of the matched filter is a time series.

The time series output will have a series of peaks whose locations correspond to the beginning of each ring. All you need to do is count the number of peaks which exceed some threshold.

If the rings could be different but you are guaranteed that they're selected from a finite set of rings (imagine different phone carriers or something) then you could use a bank of matched filters.