Understanding Class of Control of Call Manager

The core of any telephony service is to access the numbers they want to dial.
In the CUCM there are 3 main components to build a dial plan.

1. Route Pattern
If someone dials a number which matches a configured route pattern, that call is eligible for routing.
Ex:- XXX, 9.XXX, [1-2]XX, 234!#

If you want to know about Route Patterns click here.

2. Dial Plan Hierarchy
This is what tells the CUCM to route the call to the specified destination.
Ex:- route 911 calls to external gateway..

If you want to know about Dial Plan Hierarchy click here.

3. Class of Control
This controls the calling abilities of phones.
Ex:- A lobby phone should not be able to take IDD calls..

In this post, let's discuss about Class of Control.
To control access between phone numbers we use 2 things with a simple logic.

1. Partitions
2. Calling Search Space (CSS)

Partitions
Who can call me?
Name assigned to a set of numbers and patterns

CSS
To whom can I call?
List of Partitions

Let's assume following phone numbers of a company are assigned to Partitions & CSSs like below.

Let's assume 
PT_1 is assigned to CSS_2
PT_2 is assigned to CSS_1

Default is equivalent to no Partition.

You can only call a number, if that number's Partition is in your CSS & You can always call a number if that number is not in a Partition.. 

Can 1001 call to 1002 and vice versa?
Cannot.. Because  both 1001 and 1002 is in CSS_1 which has access only to PT_2

Can 1001 call to 2001 or 2002?
Can.. Because 1001 is in CSS_1 which has access to PT_2

Can 1001, 1002, 2001 or 2002 call to 3001 or 3002?
Can.. Because 3001 is not in a Partition..

Can 3001 or 3002 call to 1001, 1002, 2001 or 2002?
Cannot.. Because 3001 and 3002 has no CSS..

Can 3001 call to 3002 and vice versa?
Can.. Because both 3001 & 3002 has no CSS and not in any Partition..