The problem here is that you are upset about someone invading your privacy. First of all the burden of privacy is upon your shoulders. If the call is that important that it needs to be private that you need to take the initiative to make it private. We live in a world where the boundaries between personal space are getting smaller and smaller every day. You can't expect to walk down the street talking on your cell phone and not have someone overhear.
As for commenting on a phone call; I have done it myself. I was sitting at a bar one night with a bunch of friends and I over heard a guy talking on his cell phone next to me about needing computer services. I simply said, "I am sorry, but I couldn't help but overhear. My name is Craig, and I offer just the services that you need." And today I still do side work for his company. If I hadn't overheard or had drown it out we would have never met.
As for the volume issue, I actually tend to whisper on the phone, as my receiver is rather loud, and I don't want to overwhelm the other end of the line.
This is irrelevant. I once dated a girl that could hear what I was whispering on the phone from across my house with the TV going and all the doors and windows open. Everyone has a different level of sensitivity in their hearing, just because you can't hear it doesn't mean someone else can’t.
this is the first instance I've had of an active listener to my conversations - work related or no.
You don't know this. This is just the first time someone has said something. Maybe you're a loud whisperer and no one has ever spoken up about it.
About you specific problem, without knowing the context of the conversation or the comment that was made there is no way to make an opinion. What if you had been talking to a client and had made a major error about company product. If I had overheard that I would have told you about it, not to be nosey but to stop you from making a career-breaking mistake. But if you were talking to your spouse about a wart that you have somewhere private I would have probably kept it quiet. Although, that type of conversation is best kept to an area where you know someone isn't going to possibly overhear you.
In an open work environment you must take it upon yourself to make sure that what you don't want heard is not heard because you cannot force another person not to listen to you. Whether this is ethically right or wrong is strictly a matter of opinion in your environment.
-Al