Where in North America is area code 956?

Mexico is telephone country code +52, but there is no area code 956 in Mexico.

U.S. telephone area code +1 956 serves the southernmost part of Texas, including Brownsville and Laredo.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)