As many others, I also have a voicemail message box for my cell phone number that the caller gets redirected to if I don't answer the phone in something like 15 seconds. What I've found is that 15 seconds is a pretty short time when the phone is ringing and you need to find out where it is. It is OK if you're sitting right next to it, but if it is in some jacket lying somewhere around you're very likely to miss the call.
Therefore I called my phone company (Telenor) today to find out if it is possible to increase this timeout. I got a rather interesting answer:
Dial **61*<Your Voicemail Number>**<Timeout in seconds># on your phone, and you're done.
I don't know if it works only for Telenor customers, but it's worth a try I also found that on my Windows Mobile phone I can actually change this setting under Settings --> Phone --> Services --> Call Forwarding, so that's maybe a more general solution.
So now I've got 25 seconds to find my phone when it rings