You got it.
Depending on the box it might play a fake ring tone, or it might start the answering machine message. Either way, it's pretty putrid.
My suggestion is to order distinctive ringing, a second number that rings on the same line with a different pattern. Then get a fax machine that understands distinctive ringing (a lot of them do, check the manual) and will only answer if it hears the ring-ring pattern for the second number. That works great, fax and voice have separate numbers and there's no question who's supposed to answer what call.