https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/10_5_2/ccmfeat/CUCM_BK_ C3A84B33_00_cucm-feature-configuration- guide_1052/CUCM_BK_C3A84B33_00_cucm- feature-configuration-guide_chapter_0100010.html

A Cisco Unity Connection system call handler provides the capability to answer incoming calls, interact with callers using prerecorded greetings or menu options, and route calls based on caller input. It can also take voicemail messages if necessary.
Key Functions of a System Call Handler:
- Answering Calls: The call handler answers calls based on predefined rules, such as specific schedules (e.g., business hours, after hours).
- Interactive Voice Response (IVR): The call handler can offer options like "Press 1 for Sales, Press 2 for Support," allowing the caller to route their own call using touch-tone input.
- Taking Messages: If the destination is unavailable or if routing fails, the call handler can take messages and store them in Unity Connection mailboxes.
Reference:
https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/connection/14/administration/guide/b_14c ucsag/b_14cucsag_chapter_0111.html