One of the hardest. Try calling back and assume a bad connection. If you are still not responded to, send a letter of introduction describing your company. Call back in a week and try again. If you are still treated rudely, find another contact in the company.
On an outright “no” or just dead silence, another good tactic is to try to get some kind of dialogue going with a question that demands a “yes”. “I can understand your response, recruiters can really be annoying can't they?” WAIT, WAIT, WAIT. Then after the yes, “I am really trying to do a good position, what could I do to at least improve my credibility with you?”