You have couple of options:
1. Create a shared line appearance on all these phones and add VM DN, this will point to a single shared mailbox from all these phones
2. Create a single mailbox on CM and add all the DNs as alternate extensions in Unity under this mailbox, you will alos need to create alternate MWIs if you want the light to come on/off when there is a VM.
I personally like option 1 as it can support a lot more devices, option 2 will only be appropriate for 10 or so phones as there is a limit of alternte extensions in Unity.
In either situation there is no way to have unique password for the same mailbox.
HTH,
Chris