When a call is forwarded to the Voice Mail Pilot number (8999), Cisco Unified Communications Manager (CUCM) uses the Calling Search Space (CSS) configured on the pilot itself to route the call to the voicemail system. This CSS must contain the partition where the Directory Numbers (DNs) of the voicemail ports (or the Hunt Pilot for the voicemail ports) are located. If the pilot's CSS (INTERNALCSS in the exhibit) is missing the partition for the voicemail ports, CUCM cannot find a valid destination, and the call fails, even though the ports are registered.

A. The call forward and call pickup settings do not have a CSS on Forward All. This setting's CSS determines if the phone can reach the pilot number. The question implies the forward is initiated but fails, pointing to the next step in the call flow.
B. The Voice Mail Port partition on the CSS of the phone is missing. The phone's CSS is for calls originated by the user. A forwarded call uses the line's CSS (or a specific forward CSS) to reach the pilot, not the ports directly.
D. The CSS on the Voice Mail Pilot is missing partitions for user phones. The pilot's CSS is used for routing calls to the voicemail ports. It is not used for calls originating from the voicemail system back to users.

1. Cisco Unified Communications Manager Administration Guide

Release 12.5(1)SU1

"Voice Mail Pilot Configuration" section:

Under the description for the "Calling Search Space" field on the Voice Mail Pilot configuration page

the documentation states: "From the drop-down list box

choose the calling search space for the voice-mail pilot number. The calling search space specifies the collection of partitions that are searched to determine how to route the call." This confirms the pilot's CSS is used to route the call onward to the voicemail ports.

2. SCCP Integration Guide for Cisco Unity Connection

Release 12.x

"Task List for Integrating Cisco Unity Connection with Cisco Unified CM by Using SCCP" section:

In the step "Creating a Voice-Mail Pilot Number in Cisco Unified CM for the Voice-Mail Ports

" the guide explicitly instructs: "In the Calling Search Space field

select the calling search space that includes the partition that you created for the voice-mail ports." This directly links the pilot's CSS to the partition of the voicemail ports

and failure to follow this step results in the described issue.

3. Cisco Unified Communications Manager System Guide

Release 12.5(1)SU1

"Partitions and Calling Search Spaces" chapter:

This chapter explains the core logic: "A calling device can reach a destination device only if the partition of the destination device is a member of the CSS of the calling device." In this scenario

the Voice Mail Pilot is the logical "calling device" for the second leg of the call

and the voicemail port is the "destination device."