Buying a local SIM at a foreign airport used to be the default. An eSIM installed before you leave is now faster, usually cheaper for short trips, and means you land with data already working instead of hunting for a SIM counter at 11pm.
How an eSIM actually works
An eSIM is a digital SIM profile you install via a QR code or app, instead of a physical card. Most phones from the last four to five years support it — check your settings for 'eSIM' or 'Add Cellular Plan' before you buy one. Your regular Indian number stays active on your primary SIM slot for calls and OTPs, while the eSIM handles data.
Buying one, step by step
Pick a plan for the specific country or region (some cover multiple Southeast Asian countries on one plan, useful for a multi-country trip)
Install it on Wi-Fi before you fly — most providers require an internet connection to activate the QR code
Don't activate the data plan until you land, most eSIMs start their validity window the moment you turn on data, not the moment you install
Keep your Indian SIM as the primary line for calls and SMS-based OTPs, and set the eSIM as the data-only line
When a physical local SIM still wins
If you're travelling for three weeks or more, need a local phone number for bookings, or are heading somewhere with patchy eSIM provider coverage, a physical SIM bought locally can still work out cheaper per GB. For anything under two weeks, the convenience of an eSIM usually outweighs the marginal cost difference.