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My last 2 purchases were unlocked phones, the last one the iPhone 6. I assumed, correctly or not, that an unlocked phone would work with any carrier. In any event, I never had a problem with a sim from a local carrier of the country I was in working. I live in the US.
"Unlocked" phones don't work everywhere it really comes down to what radio chip is inside by the model number. Your best bet is to research exactly which model will support the radio bands in the countries you plan to travel and walk into an Apple Store to make sure you get the right phone.

http://www.apple.com/iphone/LTE

I did this when upgrading my iPhone 6 to an unlocked 6s to run SIM cards in China. But there was no room for error, I didn't even trust the correct number printed on the box I had to see it on the back (tiny print) of the phone to be as sure as possible before walking out of the store.

If you absolutely must purchase online I would wait until the website is updated with the option to select Unlocked (not SIM-free) and verify the model # when you receive it.
 
I gotta tell you, Apple has some real problems in educating their salespeople. I would just walk into a store but I can't deal with hordes of people, anti-social me. Anyway, the conclusion I came to with the 3rd Apple salesperson that I spoke with on the phone, the other 2 having no clue, was basically what you said, that an A1660/61 should work with any carrier when bought outright. The one thing the could not answer is if my phone would work without activation from Verizon with a T-Mobile sim. I'll just wait until the crowds die down and venture into a store.
Thanks for the very helpful info.

Understand! Here is the deal - Do NOT talk to them. You do not need to activate any sim or do anything else in store. Buy the phone full price. Grab it real tight (dont want someone stealing it!). Bring it home. Pop your sim in. Done.

Here is the secret - The "Verizon" phone comes with the Verizon sim but its basically a dummy. The phone isnt locked to Verizon so no activation is ever needed. For example: once I popped my TMo sim in, the phone shows up as a Gold 16GB Tmobile phone in the TMo online system. My phone is Silver! I just traded it in for the "free" IP7 upgrade and they didnt really care either way.

So dont waste your time talking to anyone. They will confuse you. Walk in, pay full price, walk out.
 
Are you guys 100% sure about this? The Apple site now requires account verification for Verizon, Sprint and AT&T iPhones. Wonder if the policies in-store are the same.

In the past, pre-ordering a Verizon model and paying full price wasn't ever an option so I *had* to go to the store on launch day to buy one. I don't recall being asked for any account credentials. I just told them full price and that I would set it up at home.

This year I was able to pre-order, pay full price, and keep my unlimited data and not have to queue up the night before. All in all, this year has been easy for me.
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The one thing the could not answer is if my phone would work without activation from Verizon with a T-Mobile sim. I'll just wait until the crowds die down and venture into a store.
Thanks for the very helpful info.
It'll be fine. Pop out whatever SIM is in the new phone before turning it on, pop your T-Mo SIM in and you'll be fine.
 
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