If your friend's brother wants to be creepy like this, why not just stop talking to him? He's probably enjoying watching you get so upset about this. Or why not just chat online with your friend instead of talking on the phone? Or save all your secretive and important conversations for when you see her in person.
The phone company cannot check your friends line for taps over the phone. Even if they could, your friends brother might be unhooking the tap/recorder when he's not around. Hearing clicks and pops on the line during a conversation would make sense if he's hooking a tap onto the line while you're talking.
You should take a look at the devices on
this page. Then have your friend rummage through her brother's room looking for similar devices. Or maybe just have her burst into his room while he's listening. Oh, I know! Set up a secret webcam in the brother's room to catch him in the act. That would make perfect sense!
Also, I'm very serious when I say this will help you:
http://en.wikipedia.org/wiki/Paragraph