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Sorry to nitpick but...
I looked at the DoCoMo site and found that the phone doesn't actually tell you when you're ovulating, it tells you when to expect your next period.
You input the date of your last period and the average length of your cycle and the phone predicts the date of your next period. It warns you three days in advance and on the day your period is due.
And according to DoCoMo, the fake ring isn't there to deter 'losers', it's for when you're walking alone at night so you can pretend you are speaking to someone (apparently making you less likely to be attacked).