(Pocket-lint) - Although the huge majority of phones don't support eSIM, it's only a matter of time before many smartphones adopt electronic SIM cards - essentially removing the need to have a physical SIM card (and so a SIM slot).

The first completely eSIM-only phone has now been announced in the form of the Motorola Razr flip-phone. Expect more to follow this year, but the Galaxy S20, S20+ and S20 Ultra don't have eSIM, for example.

However, the Galaxy Z Flip foldable does have an eSIM in addition to a standard SIM slot.

Phones that have eSIM support alongside a standard SIM are basically using it as a substitute for a second SIM. These still have space for a traditional micro SIM that you can use in the normal way, but you can add a second number or data contract via the eSIM - read on for more details on how this works.

The use of eSIM brings a number of advantages to device manufacturers and networks, but there are also some advantages for you, too, since you can have plans from more than one network stored on your eSIM.

So you could use one number for business and another number for personal calls or have a data roaming SIM for use in another country. You could even have completely separate voice and data plans.

But what exactly is an eSIM? And what exactly will it offer you? Let us explain more.

The term "eSIM" simply means an embedded SIM card. There are no physical SIM cards involved and no physical swapping over required by you. eSIM needs to be supported by the network or carrier and enabled by them and not all networks support eSIM as yet (see below).

An eSIM is basically a small chip inside your phone and works in a similar way to the NFC chip that's used for payment tech like Apple Pay and Google Pay.

The information on an eSIM is rewritable, meaning you can decide to change your operator with a simple phone call. They're really easy to add to a data plan – connecting devices with eSIMs to a mobile account can be done in minutes. And, as we mentioned, it was

eSIM is backed by the GSMA, the association of mobile networks and that organisation has defined the standard for eSIM worldwide.

There could be a disadvantage for consumers in terms of choice. If a phone is sold exclusively, it's possible that all handsets could come pre-loaded with a particular network rather than being open to all.

Also, people can't easily switch phones with eSIM unless they contact their network. Obviously that's not a consideration for most people but for some it will be offputting.

The days of storing numbers on SIM cards are numbered anyway for most people using Android or iOS thanks to cloud backup, but it does involve a change of mindset for those using older or cheaper phones; you'll no longer be able to swap a SIM card to a new phone physically.

Apple has turned to eSIM for the iPad Pro, Apple Watch Series 3 and Watch Series 4 and Series 5 as well as dual SIM support for the iPhone XS and XS Max and iPhone XR as well as the iPhone 11 and iPhone 11 Pro and Pro Max.

Google's Pixel 2 also supported eSIM but it was originally only used in the US for Google's Google Fi. The Pixel 3, Pixel 3 XL and Pixel 4, Pixel 4 XL do as well as well as the latest Pixel 4a.

As we mentioned, eSIM support has also come to the Moto Razr flip phone .

There's also eSIM support inside Windows 10 and some devices with cellular modems - such as Qualcomm Snapdragon-powered PCs - can use eSIM as an alternative to sticking a traditional microSIM in the slot.

eSIM is available from selected carriers. You'll either need to have a carrier's app or a QR Code you can scan. Again, the carrier will need to support eSIM.

In the UK, EE, O2 and Vodafone support eSIM, although Vodafone is currently only supporting it for Apple Watch. Three doesn't currently support eSIM.

O2 recently launched eSIM on compatible Apple and Pixel phones. O2 says that customers need only pop into your local store or call customer services to get an eSIM pack or download an eSIM.

Let's look at EE's SIM pack. With EE you can get a traditional-looking SIM pack from a store like the one pictured here. But there's no SIM in it; instead, you get instructions and a QR code enabling your device to pick up the details. Each eSIM pack comes with its own number just like a traditional SIM pack.

In the US, AT&T, T-Mobile USA and Verizon Wireless support eSIM.

Away from the traditional carriers, "global mobile network" Truphone has started selling eSIM data plans. These are able to be bought via the MyTruphone app. Truphone's international plans work across 80 countries including in Europe, the Americas and Australasia.

The app was originally available only on iOS but it is now available on Android for use with the Pixel Pixel 3, 3a and 4 (plus their XL versions). Truphone is currently offering 100MB of data completely free to new customers to try out the app.

Theoretically, eSIM should mean that you could go to another country and simply add a roaming eSIM to your handset while retaining access to your main "home" number. That's one of the disadvantages currently because, if you change SIMs abroad, for example, you can't access your own number.

"eSIMs have the potential to eliminate roaming charges abroad", says Truphone's Steve Alder. "It also enables people to switch operators quickly to get connected if they’re in an area with no signal, frees up space for new features or additional battery life and could lower the risk of device theft.

"As consumers and operators begin to see the benefits, mass adoption of eSIMs will be inevitable."

One of the advantages it offers phone makers is that we should get smaller devices because there's no need to accommodate a SIM card or the tray that holds it. There's also no need for networks to manufacture or distribute lots of SIM cards.

eSIMs will also be great for other devices like laptops and tablets, where seamless connectivity will become the norm.

Vodafone says that eSIMs will enable more connected devices simply because eSIMs don't require so much room inside a device, enabling fitness trackers or even glasses to have stand-alone 4G or 5G connectivity in a way they just weren't able to before.

We spoke to EE to ask how it will work with eSIM. The network told us that if you have a physical and eSIM provisioned and are connected to two separate networks, your iPhone will display both networks on the screen at the same time.

If the handset is in standby and both the SIM and eSIM are provisioned, customers will be able to receive calls and texts on both numbers. As you can see in the instructions below, you can then choose a “default” line that you make calls on, use SMS and that iMessage and FaceTime use. The other line is just for SMS and voice.

Alternatively, you can choose to Use Secondary for cellular data only - useful if you're abroad and using a local data eSIM.

You can store more than one eSIM in your iPhone, but you can use only one at a time.

You can switch eSIMs by tapping Settings > Cellular > Cellular Plans and tapping the plan you want to use. If you are in the UK it's Mobile Data. Then tap Turn On This Line.

If you have a QR code:

1. Go to Settings > Cellular.

2. Tap Add Cellular Plan.

3. Use your iPhone to scan the QR code that your carrier provided - you may be asked to enter an activation code.

Here's what the inside of the EE eSIM pack looks like (obviously we have blurred out the QR code):

Alternatiely, you might be asked to activate your eSIM with a carrier app:

1. Go to the App Store and download your carrier's app.

2. Use the app to purchase a cellular plan.

3. You'll then Add the Data Plan to your iPhone's settings - see how that works below.

Here's how it works in the Truphone app - select the plan you want and pay using Apple Pay. The phone's Settings app will then pick up the plan so you'll need to tap Add data plan.

You should also label your plans in Settings > Cellular. Tap the number whose label you want to change. Then tap Cellular Plan Label and select a new label or enter a custom label.

Writing by Dan Grabham.