The internal number, also known as internal pilot number, is the number the Messaging system expects to receive from the telephony server. The internal number is augmented with various prefixes when you select an E.164 telephony integration style. For more information, see E.164 Dial Plan.
The external number, also known as external pilot number, is the number the Messaging system uses in notifications and in User Preferences to inform the user how to reach the Messaging system from outside. Call data is identical for calls made to external and internal numbers.
You can define a maximum of 10 pilot numbers. You can configure each pilot number for three language choices.