tl;dr: Check the pre-paid SIM card wiki

If you want to use your smartphone to access mobile data or make calls, there are two options: either you can use your provider's own international data roaming plan (which may need to be explicitly ordered and activated, depending on your provider and mobile plan), OR you can purchase a local SIM card with a pre-paid data plan. It is almost always cheaper to purchase a local SIM card - sometimes significantly so.

You may need to first check whether your phone is locked by your provider, meaning that it has been technically blocked from accepting other provider's SIM cards. Many newer phones are unlocked by default, and in some countries (including e.g. Canada, China, Israel, Singapore) it is illegal for providers to lock your phone. If you are unsure whether your phone is locked, you can call your provider to ask, or test it yourself by borrowing a SIM card from a friend or family member with a different network provider (if it works, your phone is unlocked).

If your phone is locked, you will need to unlock it before you can use foreign SIM cards. The easiest way is to go through your provider, although in some countries they may be allowed to charge a fee to unlock your phone. Alternatively, you can search for third-party apps or websites that can also unlock it for you (usually for a fee), but make sure that the service you choose is legitimate before making any payments.

Check the international pre-paid SIM card wiki. It is an extensive database covering availability, plans, and providers around the world, including information on where to purchase SIM cards and what kind of restrictions (if any) apply.

Note that some countries are stricter than others with the sale of SIM cards, and some may require you to have a local address (in which case you can often use the address of your hotel/hostel/airbnb). You can find this information on the wiki.

If you buy a SIM card in the European Union, yes, it will work in all 27 EU countries without any extra fees. If your SIM card doesn't work automatically after crossing into another EU country, double check your phone settings to make sure that 'data roaming' is activated and that your phone is set to connect automatically to the best mobile provider (or search for and select a local mobile network manually if necessary).

Note however that many popular European countries are not part of the European Union, meaning that your EU SIM card will either not work in these countries or will charge extra fees. This includes e.g. the UK, Iceland, Norway, Switzerland, Ukraine, the Western Balkans (except Slovenia, Croatia, and Greece), and microstates like Monaco and Andorra (although in the latter, given their small size, you might still be able to access mobile coverage from neighbouring EU countries).

An eSIM ("embedded Subscriber Identity Module") is a built-in chip in some phones. Instead of a physical SIM card, which you have to buy and place in your phone, an eSIM is already embedded in your device, which means you just have to turn it on and activate it on a local network. In theory, this makes eSIMs a very practical travel option for people who own devices that support them, since you can purchase and activate your plan before you travel, without any need to buy or install a physical card. eSIM-activated devices also tend to support dual SIM, so you can keep both your home and your destination plan active at the same time.

So should you use an eSIM or buy a traditional local SIM card at your destination? Well, in most cases, the answer will still be a traditional SIM. eSIMs are still only supported in certain countries and by certain carriers. Not all devices support them. And, with a few exceptions, the plans tend to be more expensive than traditional SIM cards. Currently, you can land in almost any country in the world and easily buy a local SIM card within minutes, but eSIMs are still poorly understood and are largely niche.

For more information, see the ESIM section of the Prepaid Data Wiki.

More and more websites and apps require people to use 2FA (2-factor authentication) via their mobile devices in order to log in and validate their identity. In practice, this means that many commonly-used services, such as banking, social media, or even travel sites like transit apps, require you to be able to receive a text shortcode to your mobile phone number in order to log in.

So what do you do if you're travelling with a local SIM card that you purchased at your destination, and without access to your home phone number? There's no doubt that this is becoming more challenging than ever for travellers. Here are a few considerations, options, and workarounds:

• Use an authenticator app instead of SMS where possible. Apps like Google Authenticator, Authy, etc. work with your physical device rather than your phone number. As long as you have your actual device with you, you should be able to access these authentication codes from anywhere in the world. They're also more secure than SMS-based authenticators. Wherever you have the option, opt to set up app-based instead of SMS-based 2FA. The downside of this, obviously, is that if your phone gets lost or stolen on the road, you'll be locked out of your accounts. But at least it will work with a foreign SIM card or no mobile access.

• Set up security questions instead of SMS 2FA where authenticator apps are not options. More services are moving away from this and towards 2FA for added security, so it may not be an option anymore for many sites. But where you can, setting up security questions with random answers that only you know the answer to (i.e. make them harder for hackers to guess) can be a good security alternative.

• Dual SIM devices are increasingly common and give you the option of loading both your home SIM card and the one you purchased at your destination. Be careful, though: Unless you configure this properly, you could still incur steep roaming charges on your home SIM card. Make sure you've disabled voice or mobile data on your home SIM, and that your home provider doesn't charge you for incoming texts received abroad, unless you want a nasty billing surprise.

• Virtual phone apps like Google Voice (for Americans), Fongo (for Canadians), or other VoIP based services, will give you a virtual phone number that will work on WiFi or mobile data elsewhere in the world. However, many of these services cannot receive SMS shortcodes, or only do so inconsistently, making them hit-or-miss options for 2FA purposes. If possible, try to set up a voice call instead of an SMS text to these phone numbers, as it's more likely to go through.

• Have a trusted friend back home agree to receive your 2FA texts, and set up their phone number on everything. That person would then have to message you with the code within a few seconds/minutes of receiving it. Of course, this can be wildly impractical for many reasons: The person has to be someone you trust with your account accesses, they have to be available whenever you need to log into something, and time zones can make this tricky. This should be a last resort, obviously.