Where in North America is area code 757?

North American (USA, Canada, etc.) area code +1 757 is in southeastern Virginia, USA, including Norfolk.

Country code +7 is Russia and Kazakhstan, but there are no valid numbers beginning with +7 57.

(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, and many other countries use different prefixes.)