Since you have a business phone you probably can't use this technique, but it absolutely worked for me. When you dial a disconnected number you hear a set of tones before the message. The tones tell most equipment all they need to know. Automated dialers hearing these tones will usually mark your line as dead. If you set an answering machine to play the tones before your outgoing message the dialer will be tricked into thinking that it should take you off the list. Here is a link to a file with just the tones.
I have done this and I don't get callbacks.
http://www.scn.org/~bk269/errorbeeps45.wav