What is a dual SIM phone and can you use it with one SIM?

Dual-SIM phones are not new, and nowadays, many eSIM-enabled mobile phones have dual-SIM capability. However, dual-SIM phones are not limited to eSIMs; many dual-SIM phones also work with two physical SIM cards.

A dual SIM phone is a mobile phone or smartphone that can work with two SIM cards at the same time. Generally, dual-SIM phones are built to support two physical SIM cards or one eSIM and one physical SIM. The most popular type of dual-SIM phone is dual standby or DSDS (Dual SIM Dual Standby).

A dual SIM phone with two physical SIM slots
— A dual SIM phone with two physical SIM slots —

Dual-SIM phone allows you to use two SIMs in one phone

If you are someone who usually carries two mobile phones, then a dual-SIM phone can be a good solution for you. A dual-SIM phone allows you to have two phone numbers in one phone. Dual-SIM mobile phones are built with two SIM compartments so that you can insert or install two separate SIMs.

With the introduction of eSIM (embedded SIM), the dual-SIM capability is gaining more popularity because most eSIM phones are dual-SIM phones that allow you to use one embedded SIM and one physical SIM. The eSIM in the phone can be digitally programmed to a mobile operator of your choice, whereas the physical SIM slot requires a micro or nano-SIM card.

While the eSIM technology is becoming common nowadays in new smartphones, a dual SIM phone does not have to be eSIM capable. If you prefer a plastic SIM over eSIM, buying dual-SIM smartphones with two physical SIM slots is also possible. That way, when you travel internationally, you can purchase a local prepaid SIM card and pop it into your dual-SIM phone to have a local phone number in addition to your primary number.

I have written a dedicated post on the benefits of eSIM over a physical SIM if you need some help deciding which SIM type you want in your next phone.

Two key types of dual SIM phones: DSDS and DSFA

There are two main types of dual-SIM phones: Dual SIM Dual Standby (DSDS) and Dual SIM Full Access (DSFA). DSDS is the most common type of dual-SIM phone, and most eSIM-capable dual-SIM phones belong to this category.

With a Dual SIM Dual Standby phone (DSDS), you can have two SIMs inside your phone, and you can be contacted on either one of them. However, only one SIM can be engaged at any given time. So, if you make or receive a phone call on one SIM, the other SIM will go into standby mode.

On a DSDS phone, if someone calls you on one of the SIMs (e.g. SIM 1) while the other SIM (SIM 2) is busy, the caller will not be able to reach you and may get directed to your voicemail if you have one. However, they can call you on the SIM that is busy if you have call-waiting enabled.

Have a look at the screenshots below for a dual SIM dual standby phone (DSDS). As you may note, there are two signal bars on the phone screen which suggest that both SIMs in the phone are connected to their respective mobile networks.

— When you make or receive a phone call using SIM#1 on DSDS dual SIM phone, SIM#2 goes into standby mode —

Dual SIM Full Active or DSFA, on the other hand, allows both SIMs to be active at the same time. So if you have a DSFA dual-SIM phone and you get a call on one SIM while the other SIM is busy, you will get alerted and be able to pick up the call. But, of course, that means you may need to put one of the calls on hold, disconnect one call or switch between the two calls as required.

A dual SIM phone can also work with just one SIM

A dual SIM phone can operate using either just one SIM or two SIMs. If you only insert one physical SIM in a dual SIM phone or download and install only one eSIM on an eSIM-enabled dual SIM phone, the phone will be fully functional just like a regular phone that works with one SIM card.

Just because a dual-SIM mobile phone can handle two SIM cards does not mean that you must have two SIMs in the phone for it to function. If you insert or install only one SIM in your dual SIM phone, your phone will work just like any regular mobile phone that has one SIM. In fact, you may not notice any difference between a regular mobile phone and a dual-SIM mobile phone until you use two SIMs.

Let us look at the examples below to see the difference between using one SIM in a dual SIM phone and two SIM cards. We tested that with two mobile phones: Oppo A15 and Google Pixel 5.

In the first example, we have an Oppo A15 dual-SIM phone that works with two physical SIM cards. As shown in the picture below, the Oppo A15 dual SIM phone has two physical slots for nano SIMs.

— Screenshot of the SIM compartment of an Oppo A15 dual-SIM phone —

Your SIM can be placed in any of the SIM slots, so, for our testing, we first used SIM slot # 1 and then SIM slot #2. When you insert only one SIM card into any of the SIM slots of a dual SIM phone, it will mark that SIM as the default SIM for all cellular services, including voice calls, text messages (SMS) and mobile data.

In the picture below, the left screenshot shows our Oppo A15 phone when SIM slot # 1 is engaged. The default (SIM) card settings menu displays SIM 1 under Calling and Mobile data, meaning SIM slot 1 will be used for calls and mobile internet. While SMS (Short Message Service or text message) is not shown in this menu, it is a given that SIM 1 will also be used for text messages.

The screenshot on the right shows our Oppo A15 phone when SIM slot # 2 is engaged. As you may note, the default (SIM) card settings menu shows that SIM # 2 is used for calls and mobile data.

— Screenshot of an Oppo A15 dual-SIM phone where only one SIM slot is engaged and the other one is free—

Let us now look at our Google Pixel 5 phone, which is a dual-SIM phone, but one of the SIMs is an eSIM. We used an eSIM from Vodafone UK and a physical SIM card from O2 UK for our testing.

The left screenshot in the picture below shows our Google Pixel 5 phone with only one SIM, the eSIM from Vodafone UK. At this point, there is no physical SIM card inside the phone. As you may see, the Internet menu and the Calls and SMS menu of the phone show Vodafone UK which means all cellular services, including phone calls, texts (SMS) and mobile data, are handled by the eSIM from Vodafone UK.

The screenshot on the right shows the same phone with an eSIM from Vodafone UK and a physical SIM from O2 UK. When your phone has two SIMs, you have to select which one is the default SIM for mobile data (internet). For phone calls and SMS, you can either choose a default SIM or choose the setting that asks you to select your preferred SIM every time you attempt to make a call.

If you are new to eSIM or need some help downloading eSIM on your phone, please have a look at my dedicated post on how to download and activate eSIM on iPhones and Android phones.

A Google Pixel 5 dual-SIM phone: The left picture shows one SIM (eSIM) from Vodafone UK and the right picture shows one eSIM from Vodafone UK plus one physical SIM from O2 UK
— A Google Pixel 5 dual-SIM phone: The left screenshot shows only one SIM (eSIM) from Vodafone UK. The right screenshot shows an eSIM from Vodafone UK and a physical SIM from O2 UK —

How to know which of the two SIMs to use in dual SIM phones

When you have a dual-SIM phone with two SIMs inside, you need to choose the default SIM for mobile data so that your phone can still access mobile data (internet) when you are not connected to a Wi-Fi network.

The screenshot on the left shows our Google Pixel phone with a Vodafone UK eSIM and a Three UK (3) physical SIM. As soon as the second SIM is inserted, the phone asks you to choose the default SIM for data. For the calls and SMS (the right screenshot), the phone may choose a default SIM when you insert the other SIM, which you can change in the calls and SMS settings.

— When you have two SIMs in a dual SIM phone, it asks you to choose the default SIM for data and allows you to select your preferred SIM for calls and SMS —

If you don’t want to use a default SIM for your phone calls, you can select “Ask every time” on a Google Pixel phone or “Always Ask” or something similar, depending on your phone. With that setting, every time you dial a number or call an existing contact, a small screen will pop up asking you to choose the SIM you want to use for making the call.

— On your dual SIM phone, you can choose your preference to have a default SIM for calls and SMS or request the phone to ask you every time you make a call —

I have written a dedicated post that explains how to manage the call, text and mobile data settings on a dual-SIM phone when using your phone on two different mobile networks.

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