Hi,
This is not possible using the native scripts. But it is possible if you create you own script using the CUE editor. You can have the system transfer the call to an arbitrary extension if the caller doesn't press any option. Very good documentation here:
http://www.cisco.com/en/US/docs/voice_ip_comm/unity_exp/rel7_0/administrator/script/scripteditor.html
Please write an email to as-cue-editor@cisco.com with specific questions about the editor. Also, please notice that customized scripts are not fully supported by TAC and that this free technical service is bona fide.
Thanks,
Marcos