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Understanding Calling Search Space Usage at the Phone and Line Level

Oct. 14, 2014
Global Knowledge

For those of you who are learning to configure Cisco Unified Communications Manager (CUCM), some aspects of the configuration might seem ambiguous or confusing. One of the areas I am sometimes asked about is the use of Calling Search Space (CSS) and where to best configure it.

In CUCM, call permissions are defined by using the constructs of partition and CSS. Applying a partition to a number effectively locks access to that number. No one can call that number unless they have a CSS that contains that partition.

The CSS is effectively the “key ring” that contains call permissions. A CSS can contain one or more partitions. A CSS can be assigned to anything that can originate a call, such as a line, phone, gateway, trunk or even a translation pattern.

The point of confusion focuses on applying the CSS to the phone to define what calls an end user can make. As you explore the parameters of the phone and its associated lines, you will see that the CSS can be applied at the phone level or at the line level.

So the questions often become: Where should you apply the CSS, and why are there two places to apply it? One approach is to simply pick one of the parameters and apply the permissions there. Quite often, an administrator will pick the phone-level CSS and configure it there so that it applies to all calls made from all lines. The goal, of course, is to specify what partitions are allowed to be called. However, this approach has some limitations.

If your organization has deployed Extension Mobility or Device Mobility, then applying the CSS at the phone level will give the users unexpected call results. When configuring Extension Mobility, you configure a CSS as part of the user’s profile parameters. But when a user logs into a phone, the profile CSS is applied, by the system, to the line level while preserving the physical phone’s CSS at the phone level. Therefore, it is possible to have different permission sets as you log into different phones, as the phone-level CSS may differ by phone.

In order to properly preserve a user’s call permissions in a mobile environment, the recommended approach is to apply a restrictive (blocking) CSS at the line level (which follows the user), while applying a permissive CSS at the physical phone level, simply pointing all calls to the appropriate gateway or trunk.

When a CSS is applied at both the line and phone level, the CUCM will check the call against the line-level CSS first. If the partition of the called number is not in the line-level CSS, then the system will check the phone-level CSS. If neither CSS contains the called partition, then the call will fail.

Since the recommendation is to block calls at the line level, this is usually accomplished by creating translation patterns to match the various call types and to specify that if a call matches the translation pattern, it should be blocked instead of routed.

In this example, the desired effect is for international calls to be blocked while all other calls should be routed. The configuration would look like the following:

Translation Pattern is created to block international calls. Notice the “Block this pattern” radio button is selected.


Next, the line-level CSS of a user’s phone has the CSS that contains the blocking partition for international calls.


The phone-level CSS contains partitions that allow PSTN calls to be routed to the local gateway or trunk.


So in this example, if a user of this phone picks up the line and dials an international number, the blocking translation pattern will be matched first since its partition is in the line-level CSS. The phone-level CSS will never be searched, as the system stops processing partition checking as soon as it finds the first partition match.

All other calls at this phone would be routed to the local gateway or trunk as they would not match the line-level CSS and would continue on to the phone-level CSS. It is assumed that other PSTN calls would match a route pattern in the HQ_PSTN_pt or the NANP_PSTN_pt that would route the call out.

The benefit of this approach in a mobile environment is that the user’s restrictions, when configured in the CSS at the line level (or Extension Mobility profile), follow the user to whichever phone they log into. The user gets a consistent set of call permissions regardless of where they are.

In summary, the recommended approach for CSS application is to create blocking translation patterns in blocking partitions and apply the blocking CSS at the line level of each phone based on the users’ appropriate call permissions. The phone-level CSS should simply allow all other calls to be routed to the correct gateway or trunk.

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