At this time of year, when many of us are planning our summer vacation travels, figuring out how to stay connected while overseas needs planning as well. Data usage and phone calls could exceed the price of your flight if you’re not careful. It’s not uncommon to see cellular bills of thousands of dollars. That’s because cellular carriers and airlines are a lot alike. They both relish the opportunity to pile on extra charges.

It doesn’t have to be that way. Visitors from most other countries pay only a modest premium over their local rates when they travel. I’ll explain to you what our carriers don’t want you to know.

Short trips

First, you don’t want to use your cellphone overseas with the SIM card provided, unless it’s for just a few days where you can limit your calls and avoid using the phone for data (receiving email, using Google maps, going onto the Internet, etc.). Calls typically cost $1.49 to $3 per minute, and data is $20 per megabyte, the equivalent of sending or receiving a couple of dozen of photos. My advice is to turn off the phone’s cellular and roaming data connections and connect to Wi-Fi once or twice a day and use Skype to make calls.

Long trips

For traveling for more than a few days, you can save hundreds of dollars by using an unlocked cellphone with a country-specific prepaid SIM card that you can buy before leaving or from a cellular store in the country you visit. You’ll pay 20 to 50 cents per minute of talk time, instead of a few dollars per minute. Some don’t charge for incoming calls, while others have a small connection charge. The rates are low for both calling within the country or anyplace in the world.

Unlocking your phone

Unfortunately, all cellphones are locked if you bought them with a plan from a U.S. carrier. You can try to persuade your carrier to unlock it before leaving, but whether they do is their prerogative. I asked each carrier for its policies. Most of the responses were ambiguous, and their websites offered little or no information.

Instead, I found them eager to sell special international data plans that do offer lower rates. But beware of the gotchas. If you go over, the rates skyrocket, in one case going from 49 cents to $20 per megabyte. And if you take a cautious approach and buy a larger data package, you’ll forfeit the unused minutes, effectively still paying a higher rate. You need to sign up for it as a recurring monthly plan and then cancel it after the trip.

Most of the carriers will unlock some of their phones after 90 days, but iPhones are excluded, because it's heavily subsidized. AT&T will unlock an iPhone after the completion of your two-year contract. Those who are in the middle of a contract can get their phone unlocked by paying an early termination fee of several hundred dollars.

Verizon and Sprint will unlock their dual band models, which work on both CDMA and GSM, after 90 days, but apparently not the iPhone.

Buying an unlocked phone

If you frequently travel out of the country, whether to Canada, South America, Europe or Asia, you should consider buying an unlocked phone as your primary phone, rather than buying a subsidized phone from the carrier. You can activate it with a major carrier without committing to a contract. I recommend one of the Nexus Android phones, available directly from Google for as little as $400, which will work on T-Mobile and AT&T.

Apple now sells an unlocked 16 gigabyte iPhone for $699. It works with any SIM card anywhere in the world, and on T-Mobile or AT&T in the United States. Another option is to buy an unlocked phone along with prepaid SIM cards from a company like Telestial, rated by its users as 4.8 out 5, according to Google.

The company has served 2.5 million customers using its own network, buying bandwidth from other carriers, according to Ken Grunski, the founder of Telestial. It offers a range of services and equipment, including an unlocked quad-band phone with a SIM card for $59, a SIM card for voice and data for about 49 cents per megabyte and 49 cents per minute per call with free incoming calls, and a data-only SIM card for as little as 21 cents per megabyte. Telestial’s data SIM cards can be used in an iPad that has a SIM slot, in a MiFi card, or in a USB or external data modem.

These rates can offer a huge savings compared to the U.S. carriers' rates. How much? These are some examples provided by Telestial using the carriers’ normal rates that do not require a special plan: To send a 100 kilobyte photo, Verizon charges up to $2, AT&T $1.95 and Telestial 4 cents. Streaming one minute of music from Spotify costs $38 on Verizon, $37 from AT&T and about 78 cents from Telestial. Ten rounds of Words With Friends cost $19 on Verizon, $20 from AT&T and 39 cents from Telestial. Some of them will match Telestial rates, but only with a plan subject to the gotchas noted above. Telestial sells its products from its website (telestial.com) and in some of the airport stores.

MiFi portable Wi-Fi hotspot

If receiving calls is not important, I recommend renting a MiFi data device from XCom Global (xcomglobal.com). This creates a portable Wi-Fi hotspot that connects your computer or phone to the Internet. It costs about $15 per day for most countries for unlimited data. I’ve used it to avoid Wi-Fi charges in my hotel and to get email, access Google maps, and make Skype calls on my phone.

So, you need not pay the high rates and follow the complex rules our carriers create. With a bit of advance planning, you have some good alternatives that the carriers hope you don’t discover, giving you time to focus on your airline’s hidden charges.