When a call is directed to the first hunt group in an Avaya IP Office, and then overflows to a second hunt group and then is sent to voicemail, the call will be sent to the general catch-all mailbox. This mailbox is typically used as a default mailbox for calls that are not answered by a member of a hunt group or are not directed to a specific extension.
It's not typically sent to the first or second hunt group user mailboxes or their extension group mailboxes.
Reference:
Avaya IP Office (https://www.avaya.com/en/solutions/ip-office/)