My most unpleasant experiences have mostly involved adults shouting loudly into their phones and, seemingly, spending an hour or so doing it. Long and loud business calls are terrible - and common even in First Class before take off if the caller is either from the US or the UK (I'm British) - and phoning everyone in your family to tell them what you're doing is pretty tiresome too.

How would that business traveller feel about someone turning up and sitting in his/her office at work for an hour and just eavesdropping the conversation or making a few notes?

Lounges ought to have at least one 'quiet zone' for those who really do want to relax.

That said, French and Japanese people have a phone etiquette, whereby you go into a corner (or the space at the end of the train carriage) to make your call. The rest of the world prefers to behave as if no-one else really matters. Perhaps the answer is to join in the conversation ("the person you're talking to sounds a real slut/bast..d"), or stare at them and make notes? But a) that would probably get you a thump in the face and b) would be the exact of opposite of relaxing and get you worked up even more.