# Partitions and calling search spaces

- A partition is a bucket of patterns.
- A calling search space is an ordered list of partitions.
- The phone, the line, and the trunk can each have a CSS.
- First match in that order wins.
- A pattern you cannot see is not a failed route. It is invisible.

This is the CUCM dial plan's access control. Lobby phones should not see the PSTN partition. Emergency patterns are usually placed so that every CSS used by a phone can find them. Confirm the emergency design for your region rather than copying a lab CSS.

A phone CSS lists Internal then PSTN. The user dials a ten-digit number that also exists as a shorter internal pattern. If the shorter pattern is in an earlier partition, it matches first.

## Sources and go deeper

Independent, free, learning-only student project.

Opened 2026-10. A row marked confirm has no link, because that page was not verified.

- Cisco. [Cisco Unified Communications Manager product support](https://www.cisco.com/c/en/us/support/unified-communications/unified-communications-manager-callmanager/series.html). last checked 2026-10
- Cisco. [System Configuration Guide for CUCM, Release 15](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/15/systemConfig/cucm_b_system-configuration-guide-15.html). last checked 2026-10
- Cisco. [Cisco Collaboration Preferred Architectures](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/uc_system/design/guides/PAdocs.html). last checked 2026-10
- Cisco. [Preferred Architecture for Cisco Collaboration 15, design overview (PDF)](https://www.cisco.com/c/dam/en/us/td/docs/solutions/PA/overview/15x/clbpa15x.pdf). last checked 2026-10
- Protocol and ports. [CUCM 15 TCP and UDP port usage](https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/15/systemConfig/cucm_b_system-configuration-guide-15/cucm_m_tcp-and-udp-port-usage-12-0.html). last checked 2026-10
- Protocol and ports. [IANA SIP parameters](https://www.iana.org/assignments/sip-parameters/sip-parameters.xhtml). last checked 2026-10
- Community. [Cisco Community, Collaboration knowledge base](https://community.cisco.com/t5/collaboration-knowledge-base/tkb-p/4691-docs-collaboration-voice-video). last checked 2026-10
- Wikipedia. [Voice over IP (Wikipedia)](https://en.wikipedia.org/wiki/Voice_over_IP). last checked 2026-10
- Wikipedia. [CUCM (Wikipedia disambiguation)](https://en.wikipedia.org/wiki/CUCM). last checked 2026-10
- Cisco. Collaboration 12 SRND book. confirm. The historical Collaboration 12 SRND path opened as a product listing, not the design book. Use the Preferred Architecture index, which was opened. last checked 2026-10
- Wikipedia. English Wikipedia article for Cisco Unified Communications Manager. confirm. That title returned HTTP 404. The CUCM disambiguation page is the Wikipedia page that opened. last checked 2026-10

Room or sheet file: ../../rooms/sources/partitions-css/sources.json