validatePhoneWithCountryCode method

Validates whether the string is a valid phone number with an optional country code.

This method allows 10 to 15 digits, optionally prefixed with a country code (+ followed by 1-3 digits). It ensures that the main number consists only of digits.

Returns:

• true if the phone number is properly formatted with or without a country code.
• false otherwise.

Example Usage:

String phone1 = "+919876543210";
bool isValid1 = phone1.validatePhoneWithCountryCode(); // true

String phone2 = "0987654321";
bool isValid2 = phone2.validatePhoneWithCountryCode(); // true

String phone3 = "+1-800-123-4567";
bool isValid3 = phone3.validatePhoneWithCountryCode(); // false (invalid characters)