This Sim Card Spy Ear is basically a phone you hide in a room that you can call whenever you want, but instead of it ringing, it'll automatically pick up and let you listen in on whatever is going on. Stick it under a desk, behind a potted plant, or next to the toilet to get all the audio you need to get evidence towards your impending lawsuit, or whatever it is you need to do.
The only real issue I can see with this thing, besides the obvious legal and moral issues, is that you're gonna need to recharge the battery at some point, which could get awkward. "Yeah, hey Jill, I know we've been broken up for a few months, but I was wondering if I could just go get something from under your bed. I'll be back tomorrow to put it back. Oh, no, it's nothing you need to worry about. Say hi to Rick for me!"