Before Molly Last traveled to Greece to volunteer at a cat sanctuary, she ran the numbers for her phone. Should she get a SIM card or an international roaming plan?

A calling plan for her iPhone would set her back $10 a day through AT&T. “So my 65-day trip would have cost $650,” says Last, a retired elementary school teacher from San Francisco.

But a SIM card with a basic calling plan in Greece cost just $19 a month through Vodafone. It covered text messages, calls within Greece and WiFi.

Not surprisingly, Last went with the SIM card. She used an unlocked Huawei smartphone that she bought as a spare in Spain last year. Total savings: $612.

Like Last, many travelers are doing the math when they plan international trips. They’re weighing the convenience of a calling plan against the savings of a SIM card. So when should you use a SIM and when should you use a calling plan? And are they your only options?

“It’s good planning to figure out your cell plan before you travel,” says Jacqueline Hugo, a frequent international traveler and a smartphone app developer from Danville, Calif. If you don’t decide between a SIM and a calling plan, you could pay an expensive roaming rate. For example, if you use your U.S. phone in Europe without a plan, you’ll pay $2.05 per megabyte of data. (You use data when you check emails or browse the Web.) And that could add up.

“If a 10-year-old uses your phone and downloads or streams 10 movies while in France, that would be a surprise $20,500 bill when you get back from your trip,” Hugo says.

So what does each option entail?

A SIM card has a tiny chip used to identify your mobile phone on the network. It contains your number and other personal information. You can buy a SIM card online or at your destination and use it on another network to save money. When you buy a new SIM card, you can use your device but your phone number will change.

An international roaming plan allows you to use both your phone and your phone number when you travel abroad. American carriers typically charge a fixed rate of about $10 a day, or a monthly rate of about $60, depending on your location. But there are significant restrictions on texting and data with an international roaming plan.

Deciding between the two isn’t always easy. SIM cards can save you money, but there are hidden costs. For starters, you need an unlocked cellphone compatible with the country’s network. So if you’re traveling to Europe, you’ll need a handset that uses GSM technology instead of CDMA technology. Some phones use both systems.

How can you tell which system your phone uses? Just go to the “about” settings. If you see an MEID or ESN category, you’re on CDMA. If you see an IMEI category, you’re on GSM. And if you see both, you’re on CDMA and GSM.

By the way, the easiest way to find out if your phone is unlocked is to ask your carrier. You can also test the phone by switching out cards. If you get a “SIM unlock code” request after replacing the SIM card, your phone is locked. Your carrier may be able to unlock your phone on demand.

Another problem with a SIM card: You’re dealing with a carrier based in that country. If you experience a customer service problem, it may be difficult to find support in English.

You can sum up the argument for using a calling plan in one word: convenience.

At least, that’s what Michael McKay, a retired manager for a nonprofit organization, will tell you. “My wife and I travel to Europe once each year for a couple of weeks,” he says. “We take only my cellphone and add the $10-per-day Verizon international roaming plan. Yes, I know it isn’t cheap — but it’s trouble-free.”

Subscribing to the international roaming plan has additional advantages. Some plans work in many countries. For example, Verizon’s monthly international plan works in more than 185.

“Having the plan provides peace of mind, since the plan can be purchased on a yearly basis,” says Mahmood Khan, a professor at Virginia Tech in Blacksburg, Va., who specializes in hospitality research. “Plans can also be started and terminated based on the need.”

Sometimes the answer to the whole “SIM card or calling plan” question is: neither. For short overseas trips, many travelers simply power down their phones. In the evening, they visit the hotel lobby and use the WiFi on their phones to catch up on messages. That’s often the least expensive option.

And sometimes, the answer is: none of the above. Consider Esti Chazanow’s choice — a Pixel phone that uses the Google Fi network. Chazanow, the brand manager for a watch company in Miami, says she switched to Google Fi and stopped swapping out SIM cards on every international trip. Google charges a flat rate of 20 cents a minute for calls made outside the United States.

“SIM cards were extremely annoying, especially with having different numbers,” she says. “Google Fi is very affordable. You don’t have to make any account changes or adjustments, no SIM cards necessary.”

I’ve used Google Fi for the past three years, and I agree with her. It’s less expensive than a calling plan and far more convenient than buying a SIM card. But if I had to move to Europe, I might also consider the SIM card option.

In talking to international cellphone users, I picked up a few more useful tips. First, avoid roaming without a plan. Carriers charge a premium for international roaming. (Like that $20,500 data roaming bill. Yikes!) If you go with the SIM option, use the messaging app WhatsApp, which lets you call and receive calls from other app users at no charge, even if they’re in another country.

Don’t consider just the price of adding an international plan. Also consider the carrier’s network. I spoke with several frequent travelers who based their choice on network quality as well. They complained that the networks on some cheaper SIM cards have bad call quality and limited coverage.

So should you get a SIM card or an international roaming plan for your next overseas trip? It depends on where you’re going and how urgently you need to stay connected. If you’re on vacation, maybe you’re better off shutting down your phone and enjoying your time off.

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