What are dual-SIM phones and how do they work?

There are fundamental differences between DSDS (Dual SIM Dual Standby) smartphones like the new iPhone XS and DSFA (Dual SIM Full Active) smartphones like the Galaxy Note 9. What are these differences? Let’s take a look.

What does Dual SIM mean?

It sounds like a trivial question but it's important to go back to basics. Before talking about the differences between the various dual SIM smartphones, it is essential to understand what this term means. Dual SIM is the ability to manage two SIM cards (or two phone numbers) at the same time. They are divided into DSDS and DSFA smartphones.

Dual SIM smartphones are excellent for those who have more than one phone number, for example, private and work, or for those who prefer to experience different phone rates of two different operators to get the best from both. For example, you can have one card dedicated to calls and SMS while using the other card with a more advantageous data plan. These smartphones are also very useful for those who travel often and maybe use the local card together without sacrificing availability on their personal card.

Also, many messaging apps also support dual SIM: you can choose which SIM card you would like to use to send a message. The developers of Go SMS Pro, for example, have added dual SIM support to their application.

What changes between DSDA and DSFA?

DSDA stands for Dual SIM Dual Standby and means that the two cards installed inside the smartphone cannot work simultaneously in generating "traffic" (data or voice makes no difference). To be clear, both cards installed in a DSDS smartphone hook up to the antennas of your operator at the same time, but in case of an incoming call on Card 1, for example, Card 2 will be put into standby mode, as you were making a call yourself. Of course, the same applies when reversing the cards.

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On all dual SIM smartphones, only one card at a time can be connected to the Internet. Fortunately, there are smartphones equipped with a function called Hot Swap. Smartphones equipped with Hot Swap can choose which card to use for data connections in the device settings. For smartphones without Hot Swap, it is necessary to reverse the order of the SIM cards when you want to make the change, as only the primary card has access to the data network.

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DSFA, an acronym for Dual SIM Full Active, is a more complete but also a more energy consuming technology. DSFA smartphones can receive calls on both cards at the same time, just like the old ISDN landline phone lines. In case of an incoming call on Card 2 during a call on Card 1 you will receive a call alert and you can handle both calls at the same time. DSFA smartphones are fewer and far between and are usually only high-end devices like Galaxy Note9. DSFA smartphones usually support Hot Swap.

There are some exceptions

The world of smartphones is never simple and thus, there are some exceptions to the rule. Some smartphones, for example, can also connect the secondary SIM card to the internet in DSDS smartphones without using Hot Swap, but the card only supports the old 2G networks, as it does not have physical access to the 3G/4G modem and antenna reserved for the main card

Other smartphones, such as the Huawei P20 Pro, supports 4G and VoLTE on both SIM cards at the same time, allowing you to get the best internet browsing and calling experience on every single card even though they do not support DSFA functionality and therefore cannot receive or make two calls at the same time.

Can I use two SIM cards from two different operators?

It depends! When a smartphone is blocked by a carrier, it doesn’t accept a SIM card that comes from another provider.

Do you already use a Dual SIM smartphone? If not, are you interested in this feature? Let us know in the comments below.