AT&T International Day Pass

There is a place online to activate International Day Pass as a product for your phone. Then, when it determines you have left the country it will charge you the $10 fee if you use your phone. There is a limit on how many days it will charge that relates to your billing cycle. You may want to read the info. on the ATT website.

You should let the phone connect wherever it wants locally if you are using the day pass.

Yes, your phone will choose the strongest network that ATT has agreements with (which is most of them). It has been awhile since I used that plan (since moved on). I guess the only thing I would suggest is on days you do not want to use the plan, switch your phone to "airplane mode" that way you will not unintentionally use data. The other thing I do not know is what constitutes a "day" so far as when it starts or stops (local time?, home time?)

Make sure you set it up before you go - AT&T needs to know to expect you want to use the day pass, otherwise you will incur pretty steep roaming fees. They use different partner networks so the names will depend on what country you're in. Details from AT&T below, but as I understand it the charge is per 24 hour period and the clock starts as soon as you access data (e.g. open a web browser or one of your apps is running in background), or a text or call. There have been recent posts in the forum that said the 24 hour is based on East Coast time but I don't have personal experience with that. The more troubling issue (for me, as an AT&T customer) is that the $100 maximum charge applies per billing period - so if your trip happens to span over two billing periods you could end up spending quite a fee. I myself am going to attempt to use Google Fi on my upcoming trip and set up the AT&T just in case that doesn't go well for some reason.

From AT&T website:

With International Day Pass, you can use your AT&T unlimited plan or
Mobile Share plan while traveling abroad in more than 210
destinations. You get unlimited talk within and between International
Day Pass destinations and back to the U.S., unlimited text, and use of
the high-speed data plan that you use at home. Pay a daily fee for
each 24-hour period that you talk, text, or use data within an
included destination. The daily fee applies to each line on your
account using International Day Pass. For 1 line, it’s $10 a day.
$5/day for each additional line on the same account, used the same
day. Never pay for more than 10 days per bill.

The first time you use data, send a text, or make or accept a phone
call, your 24-hour International Day Pass begins and you will be
charged a daily fee. You won't trigger another daily fee until the
next time you use your device in an included country after that
24-hour period ends. The daily fees are calculated and charged to your
bill at the end of your billing cycle.