OK, got it, so then here is one solution...
Make a Hunt Group (lets say x567) that includes a member of x456, and has it's recall point set to a new CRA (lets say x678) and the Recall Timer to ~18 seconds, then go into the CRA and remove all greetings for day and night, and set the Timeout Value to send the call to Mailbox 456, then Call Forward x123 to x567 (the Hunt Group)
Call comes in, outside caller dials 123 (or is transferred via some method to x123), Call Forward send the call to HG x567 which rings x456 then after 18 seconds it recalls to CRA x567 which does nothing but force the call into MB 456.
Should be a done deal, then when the worker returns, just remove the CFA from x123 and all is normal again.