# Sequence of NPA/NXX Screening

Without NPA/NXX call screening, the system restricts outdials only by the number of digits to be dialed. With NPA/NXX call screening, the system restricts the outdial capabilities for a mailbox by allowing calls to be made only to certain area codes or to certain prefixes within an area code.

The system prioritizes the screening process in the following manner:

1. Removes the absorption digits (numbers at the beginning of the dial string that allow access to outside lines) from the dialing string prior to the screening process. These numbers were entered in the Digit Absorption Table, one of the RCOS menus.

2. Compares the number called to the Exact Match Table, one of the RCOS menus, which specifies whether the number is allowed or disallowed. The first table shows the screening method.

3. Counts the remaining number of digits. If the dial string does not contain an NPA (area code), it adds the home NPA.

4. If the system does not receive an exact match for the dial string, it then continues with the NPA/NXX screening.

5. Passes the NPA to the NPA Table, one of the RCOS menus, and determines if the area code is allowed or disallowed. The second table shows the screening method.

6. If the NPA contains an NXX Table, another RCOS menu, the system disregards the selection of the allow or disallow status for the area code. The determination of whether to allow the call is based only on whether the prefix is allowed or disallowed in the NXX Table.

7. Screens the number with the NXX table. The third table shows the screening method.

8. Allows the number or notifies the mailbox owner that the number is not within the mailbox owner’s calling area.

Exact Match Number Table
Is the number in the table? | If the table is to allow the call, the system... | If the table is to disallow the call, the system...
--- | --- | ---
Yes | Allows the call | Blocks the call
No | Blocks the call | Allows the call

NPA Table
Is the NPA in the table? | If the table is to allow the call, the system... | If the table is to disallow the call, the system...
Yes | Checks for NXX table. If no NXX table exists, places the call. | Blocks the call
No | Blocks the call | Checks NXX table. If no NXX table exists, places the call.

NXX Table
Is the NXX in the table? | If the table is to allow the call, the system... | If the table is to disallow the call, the system...
--- | --- | ---
Yes | Allows the Call | Blocks the Call
No | Blocks the Call | Allows the Call