Area code 406
Coordinates: 47°00′01″N 109°45′04″W / 47.00028°N 109.75111°WArea code 406 is the telephone area code covering the entire state of Montana. It has been Montana's area code since area codes were created in 1947.
As of 2013, there were 1.7 million lines served by 47 carriers; all but two wireline providers support local number portability (the exceptions are small rural independents Hot Springs Telephone Company and Northern Telephone Cooperative). Sufficient available codes for new exchanges in the 406 area code exist to meet anticipated demand until 2019; number pooling has been implemented in response to increased demand and inefficient use of 406 numbering resources.
On December 10, 2008, Google's Gmail Blog announced that SMS messages could be sent from inside Gmail. The messages are sent from numbers in the 406 area code. Each number is unique and can be bookmarked in your phone for replies or future use.
Google also employs the use of area code 406 for the assignment of reply-to or callback numbers for Google Voice's customers' contacts.
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Wikipedia