iPhones marketed as "Dual SIM" are in fact not true Dual SIM phones, but are Dual Standby phones.[1]
In a Dual Standby phone, accepting a call temporarily locks the modem to that carrier's channel, essentially disabling the second SIM for the duration of the call--however, if that second carrier supports "WiFi calling" (Generic access network), then the phone may be able to use a known WiFi network (if connected to one), or even the data of the first carrier, to keep the second line open during the call.[2]
eSIM
Furthermore, Apple began introducing eSIM technology into iPhone at the same moment that it introduced their first Dual Standby iPhones (XS/XR) in late 2018. This allows up to 8 eSIM profiles to be stored on the device, although only one or two (depending on the model) can be active simultaneously, using Dual Standby mode.
| model | Dual SIM mode |
|---|---|
| iPhone Xs/Xr iPhone 11 iPhone 12 iPhone SE 2nd gen |
1 physical + 1 eSIM |
| iPhone 13 iPhone SE 3rd gen iPhone 14 |
dual eSIM, or 1 physical + 1 eSIM |
| iPhone 14 (US model) | dual eSIM only (no physical sim tray) |
Some models sold in China can accept two physical SIM cards into the custom-built SIM tray, due to eSIM phones not being allowed in that country.[3]