The auto-attendant dial 0 position is typically defined in Mailbox 0 as it's extension... This is an extremely common hacking point and we have seen this get changed quite frequently. If mailbox 0 is a transfer only mailbox to extension 0, then you need to look at the Dial 0 reroute in form 19 of the tenant of the voicemail ports.
My guess is that the "extension" on mailbox 0 is no longer 0 or the LDN appearance. Check that first.
