iPhone 6 Initial Power Up SIM Card Locking? First, my question: Does swapping out a foreign SIM for the Sprint SIM that comes in new Sprint iPhone 6's prior to the first power up in any way prevent the iPhone from being locked by Sprint for international use. (I understand that unlocking Sprint iPhones for domestic use is impossible; I'm only asking about international use)

Now, a little background: I recently purchased an on-contract Sprint iPhone 6, and have not yet powered it on - though it was added to my account at the store already. I will be taking a trip overseas in about 3 weeks and would like to use the phone with local carriers while I'm there. Unfortunately, according to Sprint's international unlock policy, I won't be able to get them to unlock the phone before then since it won't have been active for 90 days. I've read hints online about being able to prevent a Sprint iPhone from getting locked internationally by swapping in a foreign SIM for the standard Sprint SIM when first powering it on. Is there any truth to that? Would Sprint have any way of noticing that I've powered it up this way? Would it cause any problems with my account with them?
We don't know what you wound up doing or what happened ...  

However, I will tell you that I recently obtained a Sprint iPhone 5 that had no SIM card and had never, ever been activated.  Apparently it was an Apple repair exchange, but the owner decided to move service over to Verizon rather than activating his exchange, and sold it to us instead when he found out Verizon could not activate it there.

I considered, like yourself, that perhaps the phone is in fact unlocked until the first SIM is installed.  So, I initially put in a U.S. AT&amp;T SIM card instead.  That worked, and even displayed 4G service availability on the screen, until we got to the iTunes portion of the start up when the phone tries to verify itself with Apple and get the right carrier profile.  We received a message in iTunes that the "activation policy" does not allow using this SIM card unless Sprint unlocks the phone, and "this is not a hardware issue."

In my opinion, that is a pretty good indication that the phone is locked at the factory before any SIM is inserted, and also that it is possible for the phone to run even on AT&amp;T within the U.S. if properly authorized (something Sprint denies).  As for whether the factory SIM lock includes international SIMs (your actual question), obviously we don't know from my experience, but I'd venture to guess it probably does as well, as in my opinion, the international unlock is a really just a partial SIM unlock which leaves the U.S. GSM carriers blacklisted.