Hi Dimitris,
Please find the link of some useful docs for General Delivery Mailbox below:
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/unity_exp/design/design21/cuedg21/cuevmdes.html
https://supportforums.cisco.com/discussion/10944396/how-configure-mwi-gdm-general-delivery-mailbox
Thanks
Divjot Nanda
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