Hello guys,

Today I went into a lawyer's office to take advice on employment issues. I asked the porter where I could find someone adequate for the purpose. He was nice and gave me directions. I knocked on the door tentatively and peered in to see nobody in there. I went next door, I knocked again, looked in and found a middle aged woman chatting nineteen to the dozen on the phone.
I was a bit undecided as to what to do in the first place. I thought, if I interrupted her, it would not be a very nice thing to do, but then, I did not have all day.
Finally I took the first alternative and what happened afterwards left a sour taste in my mouth. When I interrupted her, the lady looked at me scornfully, probably thinking who that presumptuous whippersnapper could be. Then continued with her conversation, not  bothering I was there whatsoever.
I closed the door and waited in the hall more than I would have if I had known the amount of thumb-twiddling ahead of me before "my man's" arrival.

Question: Is it impolite to interrupt a conversation in such a situation? What do you do in such situations?

It is just like a situation question raised during an interview. Firstly, What is the woman's role in that office? If she is a secretary, she is very rude and in bad manner to talk so long on the phone despite of a guest at presence. If she is the lawyer herself, she is not respectful for client's time. So she don't deserve client's trust especially on the legal problem. According to your description of the situation, I can't find any inappropriate behavior of yours. But for the issues to be done more smoothly, you may make an appointment in advance by phone.