IP Telephony - How to cancel background noise

Answer
Background noise that is not constant is hard to filter out. I would be very cautious of noise cancelling headsets as they are probably talking about the earphone portion. They use the microphone of the headset to filter out background noise from what you are hearing but they do not attempt to filter it out when transmitting to others.

The only microphones that I know of that are truely background noise cancelling are those used in the aviation industry. They have two microphones placed back to back within the mouth piece. One microphone listens away from your mouth and captures pure background noise. The other microphone points toward your mouth and captures only your voice with as little background noise as possible. The two signals are then fed into a "comparator" which compares them. Sounds which were on both microphones must be background noise, and so it filters that out. It leaves only the pilots voice. You can see some here:

http://www.sennheiser-aviation.com/home

If you look closely you will see the noise cancelling microphone on the "wrong" side of the mouth piece.

However, these are expensive and primarily designed to cut out wind noise and humm from the motors which is a constant "white" noise. I am not so sure how they would deal with irregular background noise.

Perhaps the best solution is to move somewhere quieter for recording sessions. Or try to sound proof the windows with double glazing and stick sound absorbing material to your walls - I have seen some people use empty cardboard egg trays stuck to their walls. Looks eccentric but works beautifully.