Dual SIM is a feature that allows a single smartphone to hold two SIM cards (or a SIM + eSIM) and use two phone numbers simultaneously. iPhones from the XS (2018) onward and many Android devices support it. It eliminates the hassle of carrying two phones, allowing multiple numbers to be managed on one device.
There are four main use patterns for dual SIM. First, separating work and personal numbers on one device - turning off the work SIM after hours blocks after-hours calls. Second, combining a domestic carrier with a foreign carrier - adding a local SIM during overseas trips avoids expensive roaming charges while still receiving calls on the Japanese number. Third, combining different plans for voice and data to optimize costs - a popular combination is an unlimited calling plan from a major carrier for voice and a high-data MVNO plan for data. Fourth, keeping a backup SIM from a different carrier in case the main carrier experiences an outage.
Dual SIM operates in two modes. DSDS (Dual SIM Dual Standby) allows both SIMs to be on standby (waiting for calls), but while one is on a call, the other goes out of service. Most devices including iPhones use this mode. DSDA (Dual SIM Dual Active) allows both SIMs to handle calls and data simultaneously - you can receive a call on one SIM while already on a call with the other. DSDA-capable devices are limited but valued for business use.
For setup, you need to designate the default line for outgoing calls, data, and SMS separately. On iPhone, go to Settings > Cellular to configure each SIM's role. You can also assign a specific calling line per contact. When switching carriers via MNP, converting one SIM to eSIM frees up the physical SIM slot. See How Mobile Phone Numbers Work for the numbering system and Number Portability Guide for switching procedures.