They never leave a message, and when we answer the line is dead.

they left a message.  the message was one of those "you're call is important please wait for the next available operator" type deals.  They called me, why would it go to an on hold message?

Showed up as a missed call on my cell; I don't call back missed calls unless it's someone I know; will repost if they call again.

I believe this may be a call center contracted by medical providers; I received another call; they just wanted to review my balance due after my insurance paid.

After a missed call from this number I got a voice mail saying "we apologize for the delay, please wait for the next available representative". Odd.

They call, I don't answer.  They don't leave a message, I don't return call.  It's annoying, but there's nothing you can do about it.  I've reported the call on my donotcall registry, but it does no good.