Dual-SIM

Some mobile phones support use of two SIM cards, described as dual SIM operation. When a second SIM card is installed, the phone either allows users to switch between two separate mobile network services manually, has hardware support for keeping both connections in a "standby" state for automatic switching, or has individual transceivers for maintaining both network connections at once.

This specifies whether a device is capable of supporting two SIM cards. The two major types of dual-SIM phones are active and standby. Dual-SIM Standby (DSS) requires the user to specify which of the two SIMs is able to make and receive calls, while Dual-SIM Active (DSA) enables both cards to receive calls at the same time. This latter feature usually requires an additional transceiver for the secondary SIM card, and as such consumes more battery life. More recent models feature Dual SIM Dual Standby (DSDS) technology which enables them to have two active SIMs with only one transceiver.

Dual SIM phones are mainstream in many countries where phones are normally sold unlocked. Dual SIMs are popular for separating personal and business calls in locations, where lower prices apply to calls between clients of the same provider, where a single network may lack comprehensive coverage, and for travel across national and regional borders. In countries where dual SIM phones are the norm, people who require only one SIM simply leave the second SIM slot empty. Dual SIM phones will usually have two unique IMEI numbers, one for each SIM slot.

Devices that use more than two SIM cards have also been developed and released, notably the LG A290 triple SIM phone, and even handsets that support four SIMs, such as the Cherry Mobile Quad Q70.

The first phone to include dual SIM functionality was the Benefon Twin +, released by Benefon in 2000. It wasn't until the late 2000s, however, when more dual SIM phones entered the marketplace and started to attract mainstream attention, most of them coming from small Chinese firms producing phones using Mediatek systems-on-a-chip.

Such phones were initially eschewed by major manufacturers due to potential pressure from telecommunications companies, but in the early 2010s Nokia, Samsung, Sony and several others followed suit, with the Nokia C2-00, Nokia C1-00 and Nokia C2-03 and most notably the Nokia X, phones from Samsung's Duos series, and the Sony Xperia Z3 Dual, Sony Xperia C and tipo dual. Apple added dual SIM support in its 2018 iPhone XS models, with models sold in China containing two physical SIM slots, and models sold elsewhere supporting dual SIM by means of Embedded-SIM alongside a single physical SIM.