I, too, have done telemarketing...but for a non-profit charity that provides help to abused children. The law concerning non-profits and the "Do Not Call" list doesn't apply. However, if someone ever told me not to call back, then , why bother...go to the next call and perhaps they will say yes. With the economy as it is today, some people have to take jobs doing telemarketing to feed their families...However, it does annoy me very much when done improperly. And, if your client says NO, once should be enough.
M-A-N-Y calls from this California # and I live in Canada. I am on a 'do not call' list in Canada, but I don't know if that applies to U.S. numbers, as well...