What are cash receipts (현금영수증) in Korea and why does every store ask for my phone number?

현금영수증 (cash receipt) is a receipt for cash payments that is linked to your identity, and stores ask for your phone number so they can register the purchase under you with the National Tax Service through Hometax. The reason it matters is taxes: for employees who do the year-end tax settlement (연말정산), a portion of your annual spending above a threshold becomes an income deduction, and both card payments and registered cash receipts count toward it, so people give their number to make sure cash spending is credited too.

To use it, you register your phone number once on the Hometax site, which has an English section, and then just give that number when a cashier asks. It is not mandatory and nothing bad happens if you decline, but if you are a tax resident who files year-end settlement, it is basically free money back at tax time, so it is worth doing. If you do not file Korean taxes, for example as a short-term visitor, the deduction does not benefit you, though the receipt still works as proof of purchase for warranties or refunds. The cashier will ask something like '현금영수증 하실래요?', and you can simply say your phone number or answer no.