This happens to be a pet hate of mine also were wondering about...

This happens to be a pet hate of mine also were wondering about "blank" phonecalls? Turns out these marketers use software to ring twenty (this number can be up or down) phone numbers at once The Marketer answers the first to answer, leaving other 19 calls to ring untill answered or drop out

(Australian Direct Marketing Association) ADMA Do Not Contact Service click link and fill out form this should stop those that are registered from contacting you (but not unregistered annoyances)

Do not give out phone numbers if you must use a mobile or a voice mail contact (Mbox offers fax and voicemail)

When you do get a annoyance call get details of product, call that product and tell them you will not use that service, If they are having a meeting explain that they better have very good security protection!Ask how they got your number. Then ask to speak to manager demand they not call again (not that unregistered marketers will listen)

Took me a while but gone from around ten a day nuisence calls to now near none (had one last week after 3 months of none)