While daydreaming today, I remembered back to this one time when I was a kid, and somehow I was able to hear my neighbor's phone conversation with my own phone. It was pretty cool! It didn't really work that much after that, but now it's got me thinking. How can I do this again and listen to people's phone conversations?

I found these instructions for modding a handheld AM/FM radio, but I'm not sure if the instructions are still good or not.
http://www.totse.com/en/phreak/bugs_and_taps/tapphon.html

Many moons ago, you could program the frequencies into a regular old police scanner, and pick up phone conversations for blocks. That ended when cordless phones left the low frequencies and got into 800 and 900 Mhz and 2.5 and 5 Ghz. Those are trunked/encrypted, it's impossible to hear them on scanner

You can still hear 900MHZ Cordless (not celluar), some Vtech. Anything above that(2.4,5-6GHZ) is digital trunked systems/encrypted.

Okay. Is there any way to decrypt signals?

No you cant.

EDIT: At least the signals you want to decrypt you cant.

No you cant.

not yet

I thought you meant the bluetooth pieces that people carry in their ears. That seems feasible, no?

not even close to true.

1. fon cloning

2. catch convo prior to encryption (ie. some encryption doesn't take place until it reaches the network).

3. catch keys in transit and decyper at will.

4. ask in Network Security & Hacking for further verification of these theories...

* i'm not saying they are necessarily practical... they are just things i would try, or look into further if i was goinng to take on this very tough project.