How to choose a prepaid mobile data plan

Data-only SIM cards can give you freedom from two-year contracts and high-priced smartphone hardware. They're also great if you're travelling internationally, or if you have an old smartphone or LTE-enabled tablet that you want to use off-contract.

These seven pre-paid SIM cards will give you plenty of options, whether you're travelling internationally, looking for talk and text or just want monthly data.

Things to know before you buy

Compatibility can get confusing when you start considering pre-paid SIM cards, but the most important thing you'll need to know is whether your device is GSM or CDMA. If you aren't sure what the difference is, this article from DigitalTrends lays it out clearly. Some devices, including newer models of the iPhone, can operate on both GSM and CDMA -- but it can get complicated depending on your carrier.

You'll also need to ensure you have an unlocked device -- meaning it's not tied to a contract with a carrier. Devices can be unlocked by carriers, which was a big win a few years ago when the FCC and CTIA implemented guidelines around the practice -- but you can also purchase unlocked devices. If you don't have an unlocked device, or one that's compatible with the SIM card or plan you're interested in, most pre-paid carriers offer discounted, refurbished devices.

Tello

Tello offers a wide range of data plans for CDMA phones -- data-only plans start at just $8 for 500MB and go up to $45 for 5GB per month. The amount of data you choose per month is the amount of 4G LTE data you can use before you get downgraded to a 64kbps data speed. Once you've hit your plan's limit for 4G, you'll also lose the ability to tether your phone.

Tello plans are compatible with any contract-free CDMA smartphone from Sprint, or from any another company operating on Sprint's network. It gets a little complicated outside of Sprint, but most iPhones, iPads and Nexus devices purchased directly from the retailer -- Apple Store or Google Play -- or through AT&T, T-Mobile and Verizon are also compatible. But it varies greatly depending on device and carrier -- you can see the list on Tello's website.

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TPO Mobile

TPO Mobile offers a range of plans, but unlike Tello, you're restricted to the amount of monthly data you purchase. They do offer an Infinity Plan, exclusive to CDMA phones, that includes unlimited talk, text and data for $60 per month; If you enroll in auto-pay, the monthly fee drops to $55. With the unlimited data plan, you'll get 50GB of free data per month at 4G LTE - which is a fair amount - after that it drops down to 3G.

TPO Mobile also operates on Sprint's network, and while other plans will work with GSM phones, the unlimited Infinity Plan only works with CDMA. The company doesn't offer a list of approved devices -- outside of any Sprint approved device -- but, on the website, you can enter your device ESN, IMEI or MEID to see if the device you want to use is compatible.

FreedomPop

FreedomPop is unique in that it offers a free mobile plan with 200 minutes of talk, 500 texts and 200MB of data per month. The SIM card itself is also inexpensive -- it's currently available for $.99. FreedomPop also offers some of the least expensive, straightforward data plans on the market. All plans include unlimited talk and text, while data starts at 500MB for $12.99 per month and goes up to 4GB for $34.99 per month. There aren't any unlimited data plans currently, but you can earn extra data every month through referrals and special offers.

You'll get a standard, micro and nano-sized FreedomPop SIM card to use across your devices. The SIM cards are compatible with "most unlocked GSM Android or iOS smartphones," and you can also purchase a global version of the card that will work in 25 additional countries. To see if your device is compatible with FreedomPop's network, you can check eligibility on the website.

[ Related story: How to use your Android device as a mobile hotspot ]

Ting

Ting offers straight forward plans that allow you to cherry pick features, but they technically aren't pre-paid plans. Ting charges you separately for voice, text and data, which are pooled across each device on your account. At the end of the month, you just pay for what you used. You can choose to just get data each month, however it gets expensive if you go above the 6GB option for $60 per month, with rates going up to $300 for up to 30GB of data per month.

Ting offers plans for both GSM and CDMA networks, claiming that over 80 percent of devices can operate on Ting's network. A SIM card will run you $9, but if you purchase a device directly from Ting, you won't need to purchase the SIM separately. If you're bringing your own device to the network, be sure to purchase the correct SIM card for your GSM or CDMA phone -- you can see if your device is compatible on the website.

Sprint Prepaid

Sprint's SIM card kit, which includes all three sizes, retails for $9.99, but with current sales, you can pick one up for $2.99. There are three pre-paid data plans to choose from. You can get 3GB for $40, 5GB for $50 or unlimited data for $60; all plans include unlimited talk and text.

Sprint's pre-paid plans are compatible with unlocked CDMA devices purchased in the US -- they don't currently support international devices. Most phones and tablets are compatible, but there are some limitations -- for instance, while most iPhone 6 models from other carriers are compatible, only the Verizon iPhone 7 model will work. You can enter your device information on the website to see if it's compatible.

T-Mobile Tourist Plan

If you're travelling to the US and looking to purchase a SIM card once you arrive, the options aren't as expansive as they are in other countries. Your best bet is the T-Mobile Tourist Plan -- priced at $30, it's a three-week plan with unlimited data, unlimited domestic and international texting and 1,000 minutes of domestic talk. Unlimited data includes 2GB of 4G LTE data before it drops down to 2G speeds. If you'll be visiting for more than three weeks, there's also a $40 plan with unlimited talk, text and data - including 3GB of 4G LTE.

You can use your own unlocked, GSM device with the T-Mobile Tourist Plan and there's also an option to purchase a tablet SIM card. To check if your device is compatible, you can enter your carrier and device name on the site. Compatibility depends on your carrier, for example, an unlocked Verizon iPhone 6 isn't compatible, but the same, unlocked device on AT&T's network is compatible.

Cricket Universal SIM Card

For $9.99 you can purchase the Cricket Universal SIM Card kit, which comes with a regular, micro and nano-sized SIM cards. Plans include anywhere from 1GB of data for $30 per month, to unlimited data for $60 per month. After you exceed your allotted data, speeds drop down to 128Kbps -- the fine print says "reduced-speed data is typically equivalent to 2G speeds." For unlimited plans, after you hit 22GB of data, you might experience temporarily slowed data during "periods of network congestion" until your next monthly plan cycle.

Cricket was absorbed by AT&T and runs on the AT&T network -- which is GSM -- but you can search for your device on the website to see if it's compatible. All unlocked AT&T, T-Mobile devices are compatible, while only some Sprint and Verizon phones are compatible.

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