Forward a forwarded mail with postfix, based on rules

How can I get postfix to forward an incoming email to one or more different local alias(es), if the address I want the rule to be based on, is not the recipient? The address I want to act on is from a forwarded mail.

It goes like this:
On external account:

• voicemail addressed to 123+voicemail@externaldomain gets delivered to the main
  myname@externaldomain account. (this combines normal text mail and mail with voice attachments)
• All mail is forwared to my server using tonline@mydomain

On local postfix server:

• Now I want to forward only voice mail originally addressed to 123+voicemail@externaldomain to my aliases for voice@mydomain and voice@myotherdomain
  That's why I can't simply use bcc_maps

• In virtual_alias_maps I tried:

  • Using virtual_alias_maps I'm able to redirect
    voice@mydomain to myvoice@mydomain, myvoice@myotherdomain
    but this includes regular mail, too.

  • So I tried a rule like
    123+voice@externaldomain voice@mydomain, voice@myotherdomain
    but this doesn't trigger.

example mail when finally received by me using the rule from my first attempt:

Return-Path: <123@externaldomain>
X-Original-To: voice@mydomain
Delivered-To: myvoice@mydomain    
From: 456 <456@externaldomain>
To: 123+voice@externaldomain
Subject: Call from 456