Dual SIM mobile phones, one form of multi-USIM devices, have become mainstream since late 2000s in many countries, which refer to mobile phones that support use of multiple UICCs [GSMA TS.37]. So far switching between multiple USIMs / UICCs is achieved by terminal side implementations while remaining transparent to mobile networks. With the current implementations, a Dual SIM 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. Enabling 3GPP system support of multi-USIM devices has benefits, while it also introduces some potential security risks.
Separating personal and business uses
Dual SIM Phone is one of the options that allow the user to have both a personal and business phone number within the same phone. Users can place their 'work' USIM or a UICC in the phone alongside a personal USIM or a UICC, so that they only have to carry one handset.
Many business phones would enable the remote control / management from the (owner) company. Security guarantee for both personal use and business use is essential. Data privacy is crucial in many regions based on regulatory requirements. Therefore, it is very important that personal usage to be separated / isolated from business usage. In addition, separation of the associated subscriptions/preferences with different UICCs/USIMs is also necessary.
Dual SIM phones make it easier for tariff hopping. For example, a user may want a separate UICC/USIM for data services only, and Dual SIM phones would allow the user to carry both subscriptions in one single phone without repeated UICC replacement. This keeps the primary number contactable with the addition of data only UICC/USIM for use on the go.
In this use case these two UICCs/USIMs have subscriptions for different services, for example the primary UICC/USIM for phone calls while the additional UICC/USIM for data services. In addition to security and data privacy guarantee for each UICC/USIM, clear separation between the associated subscriptions/preferences becomes more important.
The following definitions captured in TR 21.905
a physically secure device, an IC card (or 'smart card'), that can be inserted and removed from the terminal. It may contain one or more applications. One of the applications may be a USIM.
Universal Subscriber Identity Module (USIM):
An application residing on the UICC used for accessing services provided by mobile networks, which the application is able to register on with the appropriate security.
Concerning the related service principles specified in TS 22.101
, some are still applicable while some may require careful review, for example
In TS 22.101, clause 13.3
"Terminals and Multiple UICCs" --
A single terminal may support the use of multiple UICC (e.g. with applications like USIM and/or banking, credit card, ...). Only one UICC shall be active at a time to access a PLMN.
In TS 22.101, clause 13.1.4
"Multiple USIMs per UICC" --
The standard shall support more than one USIM per UICC even when those USIMs are associated with different home environments. Only one of the USIMs or the SIM shall be active at a given time. While the UE is in idle mode, it shall be possible for the user to select/reselect one USIM application amongst those available on the UICC. At switch on, the Last Active USIM shall be automatically selected. The Last Active USIM shall be stored on the UICC. By default if there is no Last Active USIM defined in the UICC, the user shall be able to select the active USIM amongst those available on the UICC.
The standard must not prevent the coexistence of USIM applications, each associated with different home environments on the same UICC, so long as the security problems which arise from such a coexistence are solved.
The 3GPP system shall enable the secure support of an MUSIM UE with multiple USIMs from the same or different MNOs on the same UE.
The 3GPP system shall provide appropriate security mechanisms to prevent interference between operator's service preferences of one USIM and the operator's service preferences of the other USIM.
Each USIM shall appear as a separate device to the 3GPP system.