One of my biggest concerns coming to Europe each summer is mobile connectivity: I'm here for over three weeks and I need to be connected to the Internet for both work and personal reasons. This year, I'm transitioning from international data access provided by my home carrier to much less expensive, data access from locally-purchased SIM cards. But there's a major gotcha you need to know about before you make such a move.

As a backgrounder of sorts, I have a long history with cellular access in Europe, and things have really improved over time.

In 2003, we spent a week driving around the German countryside, and to give you an idea of how different things were then, we had to rent an international-capable Nokia handset with a tiny grayscale screen from a place in East Boston; and doing that required us to find our way to their office using printed MapQuest directions. But were able to use this phone to speak with our then-five-year-old son while speeding down the Autobahn. It was amazing and freeing to do that.

Flash forward to 2007 and I'd just purchased the first iPhone so I'm among the first to travel internationally with Apple's new device. The original iPhone sported 2G EDGE connectivity and no sophisticated way of managing that connection, and AT&T offered absolutely zero international plans, so I had to turn off connectivity manually and hope I didn't inadvertently use cellular bandwidth at all. You may recall this is the summer where people returned home to the US after international trips to several thousand dollars in wireless bills. I did OK, and discussed my experience a bit in An American iPhone in Paris on my old blog.

In 2010, I was likewise among the first to travel internationally with a prototype Windows Phone that Microsoft provided me with so I could write "Windows Phone 7 Secrets." I wrote about this experience on the book's blog, and the post An American Windows Phone in Europe is probably a good place to start. In fact, it's interesting to compare AT&T's international data pricing then (a 200 MB Data Global Add-On for a whopping $199.99) with now ($120 for 800 MB of data). More on that in a bit.

So it's 2014 now. A big part of my focus these days is mobile devices, particularly smart phones, and I have three phone lines on AT&T vs. just one back in the day. I've brought several devices with me for general usage, testing, and writing purposes, and I figured I could do some combination of AT&T international data plans and locally-purchased SIMs and, combined with Wi-Fi in the apartment and elsewhere, essentially have pervasive Internet connectivity for the duration of the trip.

That was the goal at least.

It's amazing how reliant we become on this connectivity and how frustrating it is when it's not available. In my case, it's a bit more than a passing fancy, however, since I'm here working, and not just vacationing. So I need to keep up on email and other work matters throughout the day. And since I'll be out and about for much of each day, I need to be connected.

So I naturally started with AT&T, my current carrier. (Some other carriers have some interesting international options, like T-Mobile.) Looking just at data, AT&T offers three basic packages:

800 MB Data Global Add-On $120.00
300 MB Data Global Add-On $60.00
120 MB Data Global Add-On $30.00

The top two items also includes a limited amount of Wi-Fi hotspot, though that requires iOS/Android and I could never get it to work despite calling AT&T twice. No matter, as I intend to use Windows Phone exclusively out in the world when possible.

Based on the past few years coming to Europe in the summer for three weeks, I know that 800 MB is workable. You have to manage the connection—i.e. turn on Airplane Mode, or disable roaming or just turn off the data connection when you're not using it—and keep an eye on usage, which is annoying. But it's about the right amount if you're careful, and AT&T's mobile app for Windows Phone works well for checking on usage.

But I don't like being careful when it comes to wireless. What I want to do is just leave it on, have it work, and get on with life.

So I decided to belatedly examine local SIM choices. I'd only done this once before, when I bought a pay-as-you-do SIM from a service called Blau in Germany. At the time, it wasn't ideal because I wanted to keep my own phone number and be able to call and text when needed, but if I had been smart about this I could have simply set up Google Voice and not worried about such things. This year, I set up a few phones with wireless data from AT&T and then figured I could try out some local SIM options, and if I found something cheap, I'd just cancel the AT&T service(s).

There's one big issue with doing this, and I knew about it up front: You cannot use an international SIM (or any other SIM) in a locked, US-based smart phone. Most US-based smart phones are locked by the carrier, and if you want to unlock them, you need to go to the carrier hat in hand and make that request. They can take days to respond and they can deny your request for any reason they can think up. So if you are going to do this, plan ahead and get your phone unlocked before you go.

I did not do this. Not on purpose, but because it simply slipped my mind. But I at least have an unlocked Google Nexus 5, so I knew I could make it work with that phone if I had to.

Walking around Barcelona this week, I started visiting wireless shops. The best deal I found—and again, I am focusing only on data and do not care about voice or texting, both of which would be local-only anyway—was at Orange. For 10 euros, or about $13, you can get 1.2 GB of LTE data for 30 days. Or for 20 euros, or about $27, you can get 2 GB of LTE data.

$27 for what will be essentially unlimited data for the month?! Are you kidding me?

You need your passport to complete this transaction—and that was consistent at the three places I checked—so I brought that in with a few different phones to see what would happen.

Ideally, I could get my daily-use Lumia 1020 on one of these SIMs. I use this phone to take pictures each day, and to share some of them on Facebook, so 2 GB of data would be great. But the 1020 is locked, and sure enough it coughed up a warning that the carrier would need to unlock it and give me a code to do the unlocking. No problem, I thought. I'll start that process and use the Nexus 5 in the meantime.

The Nexus 5 worked immediately, albeit it at HSPA rather than 4G LTE speeds, but whatever. It's great.

As for the 1020, I knew I could get this to work. I've been an AT&T customer since I switched to the first iPhone in 2007, and I pay for three phone lines. They couldn't possibly deny me this request.

Except of course that they did. Which is why I'm recommending that you do your own unlocking before you leave on a trip and try to buy a SIM elsewhere. I'm an idiot for not doing so.

"We cannot complete your request for the following reason: You have not completed your service agreement. You are under contract and must complete the contract period before your device can be unlocked," the email from AT&T explained.

This kind of infuriated me for a number of reasons. The whole seven years/3 phone lines thing, of course. But also because the phone that is tied to that line was paid off long ago. I'm using a Nokia Lumia 1020 on the line, yes, but I didn't get that from AT&T.

Whatever. I'll deal with AT&T when I get home. For the time being, I had two choices: Just use the Nexus for connectivity and continue using the 1020 for photos and not worry about it. Or... Why not trying unlocking another phone?

My second line is currently occupied by a Lumia 1520. This actually newer than the 1020, as is the original phone that was on that line. But what the heck. I sent in the request.

Surprise. AT&T OK'd that one. For some reason.

"We have reviewed your request and confirmed that the device may be unlocked," AT&T told me. "You should complete the unlock process before porting out your number for use with another carrier."

So today I'll head down to Orange, buy another 2 GB card for 20 euros, put it in the 1520, and then just use that out in the world for photos and everything else. Why the heck not? It takes great photos—just about as good as the 1020, really—has expandable storage, and it's running the latest OS version (8.1 with Update 1) and firmware (Cyan), the latter of which further improves the camera functionality. Win-win.

(I could have optionally used the original SIM in the 1520, of course. But that raises another issue: SIM size. Where the Nexus 5 and 1020 use a now-larger micro-SIM, the 1520 uses a smaller micro-SIM. So I'll need to get a smaller SIM from Orange. You can get SIM adapters that let you go up in size, and I do have a set, but you can't go down unless you're interested in surgery.)

I'll never know why AT&T really OK's or denies these requests. But the real lesson here was already stated up front: Do this before you leave. At least you can easily speak with someone if you are denied and figure out your options.

With regards to the relative costs, 20 euros ($27) for 2 GB is obviously dramatically better than $120 for 800 MB. No one is arguing otherwise. But my employer routinely pays $200 to $500 a night for me to stay in hotels during work trips. So my theory is that asking them to pay the $120 for 3 weeks of Paul working from Europe is kind of a bargain, and is in fact reasonable. Likewise, in the context of a three week trip, paying $120 for this kind of service is comparable to a single meal in Barcelona, or any other major European city. So, yes, it's expensive in isolation. But compared to some other costs that no one would blink an eye at, it's actually not terrible.

But 20 euros ($27) for 2 GB? I'm in. And I'm sure I'll get a nice thank-you note from work for saving them so much money.