Phone blocker

[Asheville Joe]

Hi.  This is a little off topic, but I hope someone knows about it.

I share a dial up line for Internet with a couple of voice users who 
occasionally pick up the phone and knock me offline.  In the past, I saw 
a device that you could plug into any phone jack in the house that could 
tell if the phone was in use elsewhere in the house and block access to 
the line.  Does anyone know what it's called or where to get it?

Thanks.

Joe