Problem is probably poor placement of ringer button
I periodically miss phone calls because the ringer volume was turned down to zero. I'm pretty sure that the problem is with the ringer volume button placement, because it is VERY EASY to touch the ringer volume button when putting the Droid into the (Motorola) belt clip.
As a result, I try to remember to touch the ON/OFF button at the top of the Droid before putting it into my belt clip, which disables the volume button. It works, but I shouldn't have to do this.
I can think of a few ways around this, but believe that the best solution would be to have a "confirm" option displayed on the touchpad whenever the ringer volume button is touched. Once confirmed, the ringer volume button would be activated until the Droid goes on standby in another minute or two. This should only be for adjusting the ringer. If in the middle of a phone call, a "confirm" request would be a nuisance. The programming for this should be pretty simple.
I found a solution that works - at least for me
I downloaded the free RingGuard app from the Android Market and it solved my problem. Whenever I (accidentally or on purpose) press the volume rocker button and am not in a phone call, a window pops up asking for confirmation, with YES and NO buttons. This has solved my problem, which I am pretty sure was due to accidentally depressing the volume rocker switch when removing the Droid from my belt clip.
However, other people may have the same problem for other reasons, so I can't say for sure that it will work for everyone. But try it and see what happens.