If an extension is programmed as the Principal User, all you have to do is put that extension or adjunct in Cover 30 and build a mailbox.
When several phones have an appearance of a particular line, the call (I'm told by Merlinman) will ordinarily land in the mailbox of the extension with the lowest logical ID. But that scenario can be controlled by using the Principal User feature, which effectively forces the call to follow the coverage path of the assigned station.
Adjuncts get immediate coverage. So if you wanted to have a particular line immediately answered by an Automated Attendant, you might consider assigning the line to an adjunct rather than a station.
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Any sufficiently advanced technology is indistinguishable from Magix. --Arthur C. Clarke (imbellished).