Your problem is probably happening because you have forward on busy to 252. B.Krike had it right. Even though 252 might be a voice mail DN, it can only be forwarded to the voice mail DN that you get when you enter Feature 985. If you call the feature 985 DN, you`ll get in to voice mail. If you call another DN for voice mail (ie 252, most likely) you will get a busy signal. If a call comes in on the reception set lets say, (lines are set to appear and ring) and they are on the phone, it will forward on busy to 252 which will give your caller a busy signal. Change the F/busy to the proper DN.
Another thing to consider, are the telephones that have lines that are set to appear and ring. The forward no answer, will override the AA, unless the AA is set to answer after less rings than the sets. The forward on busy will override the AA unless the AA is set to answer after 0 rings. If you are wanting the AA to pick up after a few rings, you will have to make sure the AA rings are the same or lower than the forward no answer on any phone that rings on incoming calls. I would also set the forward busy to none on any of these phones that ring, or when they`re in use, the caller will go into their personal mailbox, not the AA.
I hope this makes sense. It may take a bit of trial and error, but definetly change the forward on busy to the proper DN, and that will be a start. Let us know if you need more help.
Schlum