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WIthout knowing your system exactly, or the model of it. Its going to make it very difficult to help.
So im going to take a guess, and hope this is close. I think you could have a Starplus system from Vodavi. If so, this page may help. http://www.datavoice.net/PDF/Vodavi%20UG/STS_UG.pdf
Specifically page 91..
That covers how to access the voicemail and greeting for a particular mailbox.
What may be happening, is all your calls are being routed to one specific mailbox.. So its that mailboxes voicemail that is picking up when people call.
Sift through that guide, and if it looks like or resembles your system. Then let me know.
If this doesnt appear to be your system at all, then also let me know.
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Log into JustAnswer.com, then on the Top right, click on my questions
Then, click the "reply to expert" next to my face..