How do I get a Korean phone number and mobile plan as a foreigner, and what are my best options?

To get a Korean number you go to a carrier with your passport and ARC (a Korean bank account or card helps). The big three (SKT, KT, LG U+) have more stores and English support but cost more, while budget MVNOs (알뜰폰) run through the same networks at 30% or more off, sold at convenience stores and online. If you don't have an ARC yet, some prepaid SIMs work with just a passport for short-term use. A postpaid plan needs the ARC and often a card, and the key thing is getting the number in your own name so PASS and app verification work later. Compare plans on the MOYO app.