Area code 664 in the North American Numbering Plan covers Montserrat, a tiny British Overseas Territory in the Caribbean. The 664 telephone area code, standing for "MNI," was created during a split from the original 809 area code, which was done around July 1996.
A domestic call within Montserrat is seven digits. A call from another North American Numbering Plan country (such as the United States or Canada) requires 1-664- and the seven-digit phone number, in the same format as a domestic toll call.
At one time the small island of Montserrat was able to get by with only using four-digit phone numbers internally, as the first three digits after the former 809 area code were the same for everyone (664). As of May 2011, Montserrat has seven-digit numbers beginning with 236, 349, 410, 411, 412, 413, 415, 491, 492, 493, 494, 495, 496, 664, and 724.