http://hl7.org/fhir/StructureDefinition/contactpoint-area|5.2.0

The area/zone/city code that, in some areas, may be omitted when dialing locally within the zone. This extension is used when a system wishes to designate specific parts of a phone number (and potentially place constraints on which components must be present and how they're filled in).
element_idtypes
Extension
Extension.extension
Extension.url
Extension.value[x]
Extension
Extension.id
Extension.extension
Extension.url
Extension.value[x]