Extension is not standardized in ITU recommendations, except for being defined as a series of numbers with a maximum length of 40 digits. It is defined as a string here to accommodate for the possible use of a leading zero in the extension (organizations have complete freedom to do so, as there is no standard defined). Other than digits, some other dialling characters such as "," (indicating a wait) may be stored here. For example, in xxx-xxx-xxxx ext. 123, "123" is the extension.
The carrier selection code that is preferred when calling this phone number domestically. This also includes codes that need to be dialed in some countries when calling from landlines to mobiles or vice versa. For example, in Columbia, a "3" needs to be dialed before the phone number itself when calling from a mobile phone to a domestic landline phone and vice versa. https://en.wikipedia.org/wiki/Telephone_numbers_in_Colombia https://en.wikipedia.org/wiki/Brazilian_Carrier_Selection_Code
Note this is the "preferred" code, which means other codes may work as well.
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Phone number in E.164 format, as defined in International Telecommunication Union (ITU) Recommendation E.164. wiki link: https://en.wikipedia.org/wiki/E.164