sendSms method
Sends an SMS message to the specified phoneNumber.
phoneNumber: The recipient's phone number.message: The SMS message content.simSlot: The SIM slot to use (only applicable for dual SIM devices on Android).
Throws UnimplementedError if not overridden.
Implementation
Future<String> sendSms(String phoneNumber, String message, int simSlot) {
throw UnimplementedError('sendSms() has not been implemented.');
}