This would come under the "Rube Goldberg" type of solution IF and that is IF it works.
Try putting a phantom, 500 or whatever, on one phone that does not forward correctly to the vm box but to the general greeting.
Have that phone's DID ring the Phantom.
Hit the Phantom button on the phone and do a call forward no answer to voicemail.
At the very least you will see if there is a difference in forwarding between a ringing DID and a ringing Phantom.
I would not go this route but a bunch of us are really scratching our heads for SOME solution and I believe we all feel your frustration and hope to find SOME solution that will make your customer whole and let you finish this job.
