This document covers the basics of Globalized Call Routing and the number formats used globally to implement Ingress and Egress calls. With the globalized Call routing Service Providers/Enterprise customers can establish a standard numbering plan & format for the call routing and simplify the dial plans.
Globalized Call Routing
With Globalized call routing, all calls that involve external parties are based on format with the purpose of simplifying the International multisite dial plans. All numbers are normalized as follows.
Normalized called-party numbers: E.164 format with the +prefix is used for external destination. Therefore, called-number normalization is the result of globalization. Internal directory numbers are used for internal destinations. Normalization is achieved by stripping or translating the called numbers to internally used directory numbers.
Normalized calling-party numbers: E.164 global format is used for all calling-party numbers except calls from a internal number to another internal number. Such purely internal calls use the internal directory number for the calling party number. If source of calls (users at phones, incoming PSTN Calls at gateways, calls received through trunks, and so on) do not use the normalized format, the localizes call ingress must be normalized before being routed. This requirement applies to all received calls (coming from gateways, trunks, and phones), and it applies to both the calling and called-party numbers.
Note: Except for the internal calls that were mentioned (where the destination is a directory number and in the case of an internal source, the source is a directory number), all numbers are normalized to the E.164 global format. Therefore, call routing based on the normalized numbers is referred to as globalized call routing.
After the call is routed and path selection is performed, the egress device typically must change the normalized numbers to the local format. This situation is referred to as localized call egress. Localized call egress applies to these situations.
Calling- and Called-party numbers for calls that are routed to gateways and trunks:
If the PSTN or the telephony system on the other side of a trunk doesnot support globalized call routing, the called- and called-party numbers must be localized from the global format. For example, the called-party number +494012345 is changed to 011494012345 before the call is sent out to the PSTN in the United States.
Calling-party numbers for calls that are routed from gateways or trunks to phones:
This situation applies to the phones user who does not want to see caller IDs in a global format. For example, if a user at a U.S phone wants to see the numbers of PSTN callers who are in the same area code, that user may want to see each number as a seven-digit number and not in the +1XXXXXXXXXX format.
Globalized call routing simplifies international dial plans because the core call-routing decision is always based on the same format, regardless of how the number was initially dialed and how the number looks at the egress device.
Globalized Call Routing: Number Formats
Figure describes the number formats that are used by globalized call routing and explains some commonly used expressions.
Call Routing Terminology:
The process of changing numbers to a well-defined, standardized(normalized) format. In this case, all external phone numbers are changed to global E.164 format.
The process of changing numbers to global E.164 format. Ex Because the normalized format is E.164, you normalize a called number (for ex, 4085551234) by globalizing the number- that is , by changing the number to global format (for ex, +14085551234)
The process of changing from normalized format (in this case, global format) to local format. Usually the local format is the shortest possible format that does not conceal relevant information. An example of local format is 555-1234 instead of +1 407555-1234, or 972 333-4444 instead of +1 972 333-4444 (assuming that the device where localization occurs is located in the +1408 area).
Incoming PSTN call
Call from PSTN to internal phone. Like all calls, such a call consists of two call legs(incoming and outgoing). See also call ingress and call egress in this table. On an incoming PSTN call, the incoming call leg(call ingress) is PSTN gateway to CUCM. The outgoing call leg(call egress) is CUCM to internal phone.
Outgoing PSTN call
Call from internal phone to PSTN. Like all calls, such a call consists of two call legs(incoming and outgoing). On an outgoing PSTN call, the incoming call leg(call ingress) is internal phone to CUCM. The outgoing call leg (call egress) is CUCM to PSTN gateway.
Incoming call leg - call received by CUCM
Outgoing call leg - call routed to destination by CUCM
Localized E.164 (number)
PSTN number in partial (subscriber, national, international) E.164 format.
PSTN number in complete E.164 format with + prefix.