Great SIM and value. 10GB of unthrottled data (data can still be used after) for 15 days. To extend, just use the app and pay an extra fee. It was the only Sim I could find that allowed top-up and had a 15 day option (instead of 10 and 30) so this was a no brainer. There's also free access to KT wifi via KR SIM customer service, which is a Lifesaver when you exceed the 10GB and want to watch videos on long train rides! Re VOIP calls, FB calls work.