Here is the official response from support:
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All accounts can setup a callflow of “0” and users can push 0 from any voicemail and it will ring that extension. It is system-wide, it can’t be configured on a per mailbox basis, but it would allow someone to “get out” of voicemail and go back to, say, a main menu if 0 was configured as a main menu. SmartPBX always adds extension 0 automatically if you are using that application, so this would already be setup in that case. If a given account is not using SmartPBX, they would have to build an extension 0 in Advanced Callflows.
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