Unfortunately this will never work this way... But you can just remove the line key from the phone, then add a system speed call of 7890 that points to 1234, this fulfills both sending NPA-NNX-7890 to the user at x1234 and allowing internal parties to call 1234 or 7890.
There are three "proper" ways to do this with a single mailbox...
1) In the DID Rerouting Table, add an entry of 7890 that points to 1234 (thus not using up SSC entries). People need to understand that the phone has ONE dialable internal extension 1234, and the external number is just different.
2) Change the user's extension to 7890, this fulfills both necessary requirements by eliminating 1234
3) Use the system speed call method noted above, this also fulfills both requirements by allowing the user to essentially have 2 extensions
The alternative would be to use Nupoint voicemail, in that system you can have multiple extensions associated with a single mailbox, so the user's main mailbox number would be 1234, and it would have an Alternate Extension number of 7890. This is not possible in the embedded voicemail.