My current plan is to write the local police a "come get my body" email, explaining that I've been dead for a while, then use LetterMeLater to deliver it several hours after my predicted time of death. If I succeed, it's a no-fuss way of alerting the coroner. If I fail --again!-- I have plenty of time to cancel the email being sent.
After one near-disaster during my first attempt (do not trust TimeCave!), subsequent attempts worked well --at least in that I was able to go in and cancel the notification email without any attention.
As for phoning them and then killing yourself? No, that leaves too much room for a rescue with unpleasant fallout. When they dispatch an officer to investigate, for all you know he's arround the corner from your location and will arrive in time to stop you.