In most states, it's completely legal to record a telephone conversation that you're a part of without the other parties knowing. You can even record a conversation that you're not a part of, as long as you have consent from at least one person involved in the call.
However, there are a few states that require everyone in the conversation to be aware of recordings for it to be legal. California, Florida, Massachusetts, and Washington are just several of the fourteen states that demand it. Unfortunately, since smartphone makers can't make their devices with each state in mind, they tend to do... more