That sounds to me that they're trying to replace a contact center solution, which is what it sounds they need from those requirements, with a voicemail system which can only do a basic AA.
CUC cannot play any greeting before the transfer, a supervised transfer can only play a couple messages and would keep a VM port busy while attempting to transfer.
CUC would only tell you it cannot transfer to a certain number if you configure the restriction tables accordingly, you would need to translate your dial plan from CUCM to CUC. OR disable the option to transfer to DNs not associated with subscribers, and then use a CUC user license for those who need it.
Even playing a simple prompt would require creating the specific call flow you desire, for every single DN they want to have that. Which is going to be extremely hard to manage and maintain, because CUC was not meant to do that.
HTH
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