I’m afraid I don’t understand. I thought when an extn with call forwarding enabled is called, then when the call got answered by voicemail, the call was delivered to the mailbox of the originally called party, not the one that the call was forwarded to.
As you had sowed a seed of doubt I thought I'd verify this on both 3.2(65) and 4.1(15)and when I get a chance I'll try 4.2(4). The results for the 3.2 and 4.1 were the same as follows:
1) Extn 2011 Forwarded on Busy to 2012
2011 call 2013
When another Call is placed to 2011,
there is a Call appearance on 2011
After the Timeout, the call ends up
at Voicemail, leaving a message for 2011.
2) Extn 2011 Forwarded on Busy to 2012
Extn 2011 Forward on No Answer to 2012,
2011 calls 2013
When another Call is placed to 2011,
there is a Call appearance on 2011
After the Timeout, the call forwards to 2012
After the Timeout, the call ends up at Voicemail,
leaving a message for 2011.
So I'm not sure what your problem is.