Dual SIM: two numbers in one phone

Dual SIM means support for two SIM cards (physical or eSIM) in one phone. It is handy for a work and a personal number, two operators for better coverage, or a cheaper plan for data.

Two numbers in one phone

In plain words

Dual SIM is like having keys to two flats on one keyring. You do not need to carry two phones: both numbers are right there. But usually you can only "stand in the doorway" of one flat at a time: while you are on a call on one number, the other usually cannot be reached.

How it works

The phone has two SIM slots, a slot and an eSIM, or two eSIMs.
Both SIMs wait for calls at the same time (DSDS mode).
During a call on one SIM, the other is usually unavailable.
In settings you choose which SIM to use for calls, texts and mobile data.
A hybrid slot takes either a second SIM or a memory card.

During a call, the second SIM is usually unavailable

Where you'll see it

Most Android phones have two SIMs or a SIM and eSIM; iPhones have a SIM and eSIM or two eSIMs. Specs say: "2 × nano-SIM", "nano-SIM + eSIM" or "hybrid slot".

Does it matter when buying

It is very useful: two different operators give you more reliable coverage, especially during power outages when some towers work poorly. If you also need a memory card, check that the slot is not hybrid.

Myths

Myth: Two numbers mean double the battery drain. Reality: Drain is slightly higher, but not double.
Myth: Both SIMs can be on calls at the same time. Reality: On most phones, no - only one active call.

FAQ

What is a hybrid slot?

A second slot that takes either a SIM or a microSD card, but not both.

Can I have two WhatsApp accounts?

Many Android phones have a "dual apps" feature.

Do both SIMs work on 4G?

On modern phones, yes - both can use 4G/5G.