code: func IsRussianMobileNumber(phonenumber string) bool {
  	// Russian phone number has a 1 digit country code, 10 digit national significant
  	// number, and a '+' sign as we accept only international numbers in E.164 format
  	// TODO: possibly more fine grained verification
  	if len(phonenumber) == 12 && strings.HasPrefix(phonenumber, "+7") {
  		return true
  	}
  	return false
  }
docs: // IsRussianMobileNumber checks if a phone number is a russian mobile. phone numbers need
  // to be passed in E.164 format (leading '+'').