There’s an easy way to get zero roaming fees, inexpensive internet service and cheap phone calls when you travel. The trick is the SIM card, a tiny, removable chip, inside your phone.

What is a SIM card?

Subscriber Identity Module cards are roughly the same size, and indeed look like, the microSD memory cards you’d find in a digital camera. (Some phones actually have both microSD and SIM cards, but most just have the latter.) The main purpose of a SIM card is to store a small amount of data that lets the closest cell tower know, among other things, what wireless company you’ve subscribed to. For example, if you’re on Verizon, the SIM card will let Verizon’s tower know that, yep, you’re on Verizon.

When you’re traveling, this creates a problem. When your phone connects to the tower in a different country, the tower effectively says, “I don’t know you,” and either denies you service, or tells Verizon you’re roaming. If the latter, you get whatever speed Verizon and that provider have determined is acceptable, and you get charged exorbitant roaming rates. AT&T works similarly, Sprint, T-Mobile, and Google Fi are different, but we’ll get to them later.

If you’re wondering if you can just get a different SIM card that will work on a different network, you can.

For years that’s how I’ve gotten cheap data in dozens of countries all over the world. I’d arrive in a city, head to a local cellphone company store (like Vodafone or Orange), and buy a SIM and a month of service. These would have different names, like “pay-as-you-go” or “prepaid.” But in every store I’d just say I was traveling and wanted a SIM for a few weeks, and they all knew what I was looking for. Put the new SIM in your phone (make sure you don’t lose your old one, you’ll need it when you go home!) and you’re all set. If you’re not sure how to do that, the store will probably do it for you. You can be in and out with cheap high-speed data in less than 30 minutes.

How much does a local SIM cost?

The prices vary depending on country and provider, but on average I’ve paid around $20 for a month’s worth of 4G data and calls in that country. You read that right. Most roaming packages charge that for a few days, and are slow and data­-limited as well. Most companies want to look at your passport, so make sure you bring it. A handful of countries have other requirements, so a quick Google search before you go is a good idea.

The catch, and you knew there’d have to be one, is that your phone has to be unlocked. This means your cellphone company has to enable a setting on your phone that allows you to use it on a different network. The process to do this varies, but in most cases you need to have paid off your phone. The actual unlocking process is free, though.

Not every phone will work in every country, but most new phones should be fine. Best to check with your current cellphone company to verify that yours will work, and if it needs to be unlocked.

A handful of phones have dual-SIMs, which is exactly what it sounds like: two SIM card slots. This lets you keep your home SIM and your “away” SIM in the same phone. Not necessary for infrequent travelers, but for anyone who regularly goes to another country, quite handy.

Times are changing for SIM cards, however. Both Google and Apple have “eSIMs” in some of their phones and tablets. These embedded-SIMs are a virtual SIM card, and work across multiple cellular networks. This is how Google’s Fi works.

Speaking of Google Fi, it, along with Sprint and T-Mobile, have generous free international coverage. Generally, if you have one of these providers, you’ll be able to step off a plane just about anywhere in the world and your phone will work more or less normally. Google Fi has 4G data; Sprint and T-Mobile have far slower 2G, but offer 4G packages for a few dollars a day.

Will I have the same phone number?

No. Your apps will work, but you will have a new number. Your old number will go direct to voicemail until you return home and swap in your original SIM, which will return your phone back to normal. Just something to keep in mind.