The constant bip, bip, bip... one I was able to isolate pretty fast, but another took me fully 10 minutes. Every two minutes: Bo-up! Is it the headset? Not sure. Is it something I am hearing through the headset, like the GPS? Doesn't seem to be. Is it this radio? Mm... no. Is it the other radio? Yes! It's a low battery warning.
My phone also spends a fair amount of time beeping in a seemingly random manner. Some seem to have simply have no on-screen notifier. Sometimes this notice is hidden, as when in a browser, and even worse the timer for the notification expires even though it is not visible; by the time I get to the idle screen, there's nothing to see. Sometimes the phone is in another room, and I can't tell if a beep is from the phone, an alarm, the microwave, the stove, a computer, etc. etc.
A few tones are pretty unique and designed to elicit the right emotional response; sad and dying means Alison's phone battery is indeed sad and dying. Pop it on the charger. But far too many are essentially arbitrarily different tones, and very similar to other, unrelated tones.
Increasing use of multi-tasking mobile devices, and always-on applications – like a lot of the contextual ideas we come up with around here – will add to this confusion if the current trends continue. So, when you next design or specify alerting tones, put some extra thought into it; consider not just the application, but the entire device, and the whole electronic ecosystem in which your user operates.