Where is telephone country code 1 315?

Telephone country code +1 is North America, including the USA, Canada, and several Caribbean/nearby islands. Area code +1 631 is Suffolk County, New York, roughly the eastern half of Long Island.

(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 Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)