Hi Robert,
In Unity 5.x and above you can config Unity to use the "Last Redirecting Number" on a Call Forward instead of "Originally Dialed Number"
For Unity 4.x Versions;
For this configure a Voicemail profile (in CCM) for the User B and apply it to User A's DN that will allow this type of Call Forward to the desired mailbox.
Or in Unity set up Alternate Extensions so that User A is an Alternate Extension for User B etc. Sharing a Cisco Unity Voice Mail Box between Two or More IP Phones
Configure Alternate Extensions
Open the Unity System Administrator web page.
Navigate to the subscriber's profile. Select Subscribers > Find and Select a Subscriber > Enter Subscriber Information then click Find and click the Subscriber's name for the subscriber that owns the primary phone.
When the subscriber page comes up, select the Alternate Extensions option and click Add.
Enter the alternate extension number and click the Save icon.
From this good Unity doc;
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_configuration_example09186a008015ceec.shtml#steps
Route Forwarded Calls by the First or Last Redirecting Number
Cisco Unity supports the option of routing calls based on either the first or last redirecting number when a call is forwarded to Cisco Unity.
Note the following:
This option requires Cisco Unity-CM TSP 8.1(2) or later.
This option is not supported by integrations through PIMG units.
This option can be changed through the Advanced Settings Tool (AST), which is available in Tools Depot.
http://www.cisco.com/en/US/docs/voice_ip_comm/unity/5x/release/notes/501curelnotes.html#wp507371
Call Information Exchanged by the Phone System and Cisco Unity
The phone system and Cisco Unity exchange call information to manage calls and to make the integration features possible. With each call, the following call information is typically passed between the phone system and Cisco Unity:
•The extension of the called party.
•The extension of the calling party (for internal calls) or the phone number of the calling party (if it is an external call and the phone system supports caller ID).
•The reason for the forward (the extension is busy, does not answer, or is set to forward all calls). There is also a reason code for Direct Calls.
Cisco Unified Communications Manager SCCP and SIP trunk integrations can also provide the following call information (the choice of first and last redirecting number is set in the Advanced Settings Tool, which is available in Tools Depot):
•Called number
•First redirecting number
•Last redirecting number
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Note Cisco Unity can use either the first redirecting number or last redirecting number, depending on the setting in the Advanced Settings Tool, which is available in Tools Depot.
http://www.cisco.com/en/US/docs/voice_ip_comm/unity/5x/design/guide/5xcudg060.html#wp1040786
Hope this helps!
Rob
