Area code 664 is telephonearea code of Montserrat within the North American Numbering Plan (NANP). The 664 area code, standing for "MOI," was created during a split from the original 809 area code, which was done around July 1996.
When in Montserrat, use the seven digits alone. When calling to Montserrat from anywhere in the United States or Canada simply dial 1-664- and the seven-digit phone number.
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.
While Mexico also has an area code 664, which covers the municipality of Tijuana, Baja California, the Mexican area code 664 is part of the Mexican country code 52. (Wikipedia articles using the format "Area_code_NXX" are reserved for area codes in World Numbering Zone 1, which includes Canada, the United States, and certain Caribbean and Pacific islands, as shown in the chart below.)
Telephone numbers using the Mexican area code 664 may be encountered in Southern California, considering the strong cultural and economic ties between Southern California and Baja California. However, calls from Southern California to the Mexican area code 664 must be dialled as 011-52-664-XXX-XXXX. Calls to Montserrat are dialled from Southern California the same as to any other North American Numbering Plan Area Code: 1-664-XXX-XXXX.