Welcome JJ and confusion,
" Can/should we unlock our own phones? I understand you can buy "unlocking codes" for $25 or so. Is this the way to go, or is it better to hit ebay for a new unlocked phone? How do we "relock" it when we come back?"
I havent done my own unlocking since they came unlocked or using a regional sim 'unlocked" it.
You may want to ask your provide how much it will cost for you to "unlock" your phone, or ask if they have "loaners". i believe someone mentioned something about AT&T and loaners, but i would ask.
"locking" them keeps you from using some other less expensive provider, whoever that maybe. if you were to "unlock" it i dont see why you would want to "relock" it after doing so.
I would ask, WHY do you want to use your phones over there?
"If we unlock our phones, we need a European SIM card. OK. When we put that in, do we have to do anything else?"
Yes. you may need to do some minor "setups" or not.
"Will the phone now work just like it does in the states and on our same phone plan (I know the number is different, but I don't get how we'll be charged for data - still through AT&T, or do we have to sign up for a European service?)"
Your phone will work just like it has before, but with a local phone number. If you choose to buy an AT&T sim, i have no idea on what they offer.
If it was me i would buy a Pay As You Go (PAYG) sim. there is a limited number of minutes you have when you first purchase it and you can add minutes as you see fit. Most sims will come with some minutes on it and maybe some data. Whats nice in my opinion, is that you can buy data time when you want it. My UK/NL sim i can buy data time in 1 day, 3 day and 1 week blocks. It comes down to MB chunks, but i dont have to buy a monthy/yearly plan to get data as i do in the USA.
" We plan to do most of our voice/data usage over wifi. However, we both want the security of being able to use data if we need to. Is this the best option for us, or should we plan on getting the (expensive!) AT&T international plan?"
you may want to check with AT&T regarding this.
take a look at carphonewarehouse.com. look at the providers and rates
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they use to have a table with all of the providers and rates, but i cant find it now. But you can look at the individual providers and get an idea on the costs.
Just a comment. If you do buy a sim overthere, make sure you can add minutes in and out of the country you purchase it in. especially outside of the country. If you dont plan on using it further, then it isnt a big deal. the reason is that my UK sim i can not add minutes without a voucher. i have to buy it there and use it later when i need it. The catch, you have to buy it in the UK. My NL sim i can do online using paypal.