In Office Communications client select

Call Forward Settings from the drop down

At the bottom of the Call forwarding settings

Select the check box for

Only Apply these settings during my working hours specified in Outlook

Once the check box is selected define where the Unanswered calls should be forwarded to

Setting Up Working Hours in Outlook 2007

1. Open Outlook.
2. On the Tools menu, click Options.

 Click Calendar options.

1. Under Calendar work week, in the Start time and End time lists, select the start time and end time of your work day. This is primarily used to check your free/busy time availability for meeting requests from other people.