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.
While Mexico also has an area code 664, which covers the municipality of Tijuana, Baja California, the Mexican area code 664 is part of Mexico's country code +52. 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.