extension property
The phone number's extension.
The extension is not standardized in ITU recommendations, except for being defined as a series of numbers with a maximum length of 40 digits. Other than digits, some other dialing characters such as ',' (indicating a wait) or '#' may be stored here. Note that no regions currently use extensions with short codes, so this field is normally only set in conjunction with an E.164 number. It is held separately from the E.164 number to allow for short code extensions in the future.
Implementation
core.String? extension;