to fully import everything is far too painful for words.
If the customer wants to preserve certain important VM messages, one option is to set up VM to email on the existing embedded system, then listen to the VM in the mailbox and forward to email ( 6 then 1 if you're using IPO mode in embedded). At least you would have the VM sent as .wav file saved as an email attachment.
In theory, you could get the message back into your VM pro mailbox by playing the thing on your PC, on speakers, while leaving yourself a message on your phone ;-)
which gets back to the too painful for words bit.
GB