I don't know if it's illegal or not, but I think recording anyone with out their concent is just not a cool thing to do.
In New York, is it illegal to record a conversation on a cell phone or any other device without consent?
most states it is illegal
That depends ...
1 - You need to check the actual state law. In some states, as long as at least one party is aware of the recording it is legal.
2 - If they are talking on a cell phone in a public area, and you are just recording without being hooked to the phone ... It is legal. ANY conversation in a public area has "no expectation of privacy". So if a passerby could hear both sides of the conversation ... That's the phone users problem.
3 - It IS illegal to use a scanner to pick up cell phone conversations, under Federal law. HOWEVER, it is also technically illegal for cell phones to use encryption under Federal law - It's just not enforced. And you can still legally purchase older scanners that were made prior to Federal requirements to "block" those cellular frequencies.