How to Use Your Smartphone Overseas - Making Sure It Works, and Avoiding Unexpected Bills

We will soon be testing this.  We will be turning off our data, and using only when we have a WiFi connection. An easy way to do this is use the airplane mode.  If we need to use when we don't have a WiFi connection, we've signed up for Verizon's plan of $10/Day when we use the phone.  The charge only kicks in on the days we use our phones without WiFi.  We are also bringing a phone along that we will use with a sim card purchased over there.  If we need to use GPS or make local calls we'll use this phone.  

This is a particularly thorough article on the subject from Trip Savvy

Are you planning to use your smartphone while traveling internationally? Here are five simple ways to ensure a straightforward experience while you’re away, and avoid nasty bill surprises when you get home.

Make Sure Your Phone Will Work in Your Destination
First, ensure your phone will work in your intended destinations. Cell companies around the world use different technologies and frequencies, and there’s no guarantee your phone will work with all of them. Older Verizon and Sprint phones, in particular, can be problematic.

First, check the phone’s user manual. If it is marketed as a "world phone", or supports quad-band GSM, it should work in much of the world. If you purchased your phone from your cell company and aren't certain if it will function overseas, contact customer support.

Most recent smartphones should support calls, texts, and at least 3G data in many parts of the world. It's less likely that you'll have LTE/4G service everywhere, but there's a reasonable chance you'll get it at least in the same continent or region you purchased your phone.

Most cell companies don’t enable your account for international roaming automatically, due to the high costs that can be incurred. Once you know your phone is capable of working in a particular destination, be sure to contact your cell company to enable roaming on your account.
More information:

• Verizon
• Sprint
• AT&T
• T-Mobile

Check for International Roaming Packages
Using your phone overseas can be a very costly exercise. Many cell plans don’t include any calls, texts, or data while traveling internationally, and casual usage rates can be extremely high. It’s not unusual to hear of people returning from a one or two week vacation and receiving a bill of thousands of dollars for their cellphone use.
To avoid this happening to you, check to see whether your cell company has any packages designed for international use. While many such packages are still expensive compared to using your phone at home, they’re usually much cheaper than "pay as you go" rates. Affordable roaming packages are often available for travel in Canada and Mexico in particular.

Find Out If It’s Unlocked
If you’d prefer to avoid roaming charges entirely, you can do so with an unlocked GSM smartphone. With one of these, you can remove your existing cell company’s SIM card, and replace it with one from a local company in your destination.

Depending on where in the world you’re going, the card itself will cost a few dollars, while $20 worth of credit will usually give you enough calls, texts, and data to last at least a couple of weeks.

Unfortunately, if you didn’t pay full price for your phone, it may not be unlocked. There are exceptions, though, and it's getting easier to buy an unlocked phone or get it unlocked after purchase than it used to be in the United States. Most iPhone models, for instance, have a SIM card slot that's unlocked for international use, no matter which company you bought it from.

If you're not one of the lucky ones, it’s worth contacting your cell company to see if it will unlock your phone for you, especially if it is no longer under contract. Some carriers have even started doing this automatically once a phone goes off-contract, and most of the rest just require you to fill out a form on a web page.

Turn Off Cell Data (and Use Wi-Fi Instead).  If your smartphone isn’t unlocked and you don’t have a good international roaming package, there are still ways to avoid spending a fortune.

The most obvious is to turn off cellular data before you board the plane to your destination, and leave it that way until you get home. At rates of up to $20 per megabyte, you could have spent hundreds of dollars downloading email before you’ve even got to the baggage carousel.

Instead, limit yourself to using Wi-Fi while you’re away. Most accommodation now includes wireless Internet, free or at a relatively small cost, while cafes and restaurants can fill in the gaps when you’re on the go. It’s not quite as convenient as having cellular data at your fingertips, but it’s a whole lot cheaper.

Use Google Voice, WhatsApp or Skype Instead of Making Calls
Finally, whether you’re using Wi-Fi or cellular data, consider using smartphone apps like Skype, WhatsApp, or Google Voice when you need to stay in touch with friends and family back home. Rather than paying high international calling and text rates, these apps let you talk and send texts for free or cheap to anybody around the world as long as you've got a reasonable internet connection.

Using Google Voice lets you call and text most US and Canadian numbers at no cost, and any country outside that for a small fee. Skype also has low per-minute rates for calls and texts, and both apps let you call other users of the service for free no matter where they are.
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WhatsApp lets you text and/or call any other user of the app at no charge, as does iMessage, Facebook Messenger, and several other services.
With a little preparation, heading overseas with your smartphone doesn’t have to be a difficult or expensive proposition. Have fun!