Turning Mobile Phones Into Eavesdropping Devices

I'm sure I've read about this before, but I can't remember exactly where, or what the context was. Apparently, increasingly popular in Asia is a special chip that will let someone eavesdrop on the owner of a phone. Of course, you need to know the person pretty well for it to work, because you need to put the chip into their phone without them knowing (which explains why it's apparently popular with suspicious spouses). What the chip does is allow someone to call the phone via a specific phone number - and the phone will immediately answer without ringing - allowing the caller to hear whatever is happening near the phone. I wonder how useful this actually is. First, you have the problem of installing the chip without the person realizing, which limits who you can use it on. Next, you have to know exactly when to call, and know that they'll have the phone nearby. Also, I wonder how it deals with the situation where the person is already on the phone - would it automatically answer the call waiting call? That could get embarrassing! What if the owner of the phone tries to make a phone call while the person is already eavesdropping? Will they see that the phone is already in the middle of a call? It seems like there are a lot of reasons why this is unlikely to be very useful except in a few very specific situations.