Which CSS is evaluated first for an off-net call?

• A. Route CSS
• B. Call CSS
• C. Line CSS
• D. Device CSS

Answer: (C)

When making an outbound off-net call, CUCM uses a layered access control called the Calling Search Space to decide if the call can be routed at all. The first gate the system applies is the Line CSS, because it defines which partitions (and thus which route patterns) the specific line can reach. If the line’s CSS does not include the partition that contains the external route pattern or trunk you want to use, the call is blocked right away, regardless of other settings.

If the line CSS allows access to some route patterns, the next step is to evaluate the Route CSS to determine the exact route pattern to use for dialing the outside destination. Only after the line CSS passes do these more specific patterns get considered. Device CSS governs what the device as a whole can access, and Call CSS controls other call restrictions, but for an off-net outbound call, the line-level permission is the first, critical filter that determines whether the call can proceed.