SNMP Master Agent serves as the primary service for the MIB interface. You must manually activate Cisco CallManager SNMP service; all other SNMP services should be running after installation.
Reference:
https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/service/9_0/admin/CUCM_BK_C13
6FE37_00_cisco-unified-serviceability-administration-90/CUCM_BK_C136FE37_00_cisco- unified-serviceability-administration-guide_chapter_0101.html

A Cisco Unity Connection Call Handler is a software application that answers calls, plays greetings, and routes calls to users based on caller input. Call handlers can be used to create automated attendants, voice menus, and other interactive voice response (IVR) applications.
Call handlers are created and managed using the Cisco Unity Connection Administration interface. When creating a call handler, you can specify a variety of settings, including the greeting that is played, the caller input options that are available, and the destination that calls are routed to.
Call handlers are a powerful tool that can be used to create a variety of IVR applications. By using call handlers, you can improve the efficiency of your organization's communications and provide a better experience for your callers.
Here are some additional tips for using call handlers:
Use call handlers to create automated attendants that can answer calls and route them to the appropriate person or department.
Use call handlers to create voice menus that can provide callers with information or options.
Use call handlers to create interactive voice response (IVR) applications that can collect information from callers and process their requests.