The entire state of Montana is in area code 406, which is +1 406 in international format.
The code +1.406 is used throughout Montana.
The 307 area code serves the entire state of Wyoming - where the majority of Yellowstone is located. Note, however that Yellowstone National Park stretches a bit outside the boundaries of northwest Wyoming into Montana (area code 406) and Idaho (area code 208).
Most numbers in the 406-861 prefix in Billings, Montana, belong to Cingular Wireless, which is now part of AT&T. However, with number portability, any specific number could have been switched to a different cellular carrier, or even to a landline.
The area of Sarpsborg is 406 square kilometers.
The area of Murgul is 406 square kilometers.
{| |- | 334 406 |}
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'00' isn't part of the country code, it is a commonly used international access prefix. Country code +1 is North America (USA, Canada, etc.), and +1 406 is the U.S. state of Montana.
GEICO
The area of Jesús de Otoro is 406 square kilometers.