I had big trouble in switching between US and Korean MVNO serviceㄴ with my iPhone. Finally I made it work but the success came after about a week of crazy Google search and numerous attempts that turned out to be useless, probably because my case was highly unusual. I am thus writing a post, hoping that my story would help a lot of people. My problem occurred in carrier-unlocked iphone 6S a1688 model. The MVNO carrier was Tracfone on US, and Annex telecom (www.annextele.com) in Korea. Supposedly a programming bug by Tracfone so I believe my issue has nothing to do with detailed model of iPhone or overseas MVNO carrier. Or you could have similar problem with other carriers than Tracfone with other bug.
[Symptom 1] For convenience, I will refer to the two iPhones as PH1 / PH2, and the two Annex SIM cards as KS1 / KS2. KS1 worked nice (despite on iPhone 5S) two years ago, and KS2 is the one I purchased new in Jan 2018. On PH1 call and text worked fine with no cellular data. On PH2 data worked too (PH2 stopped working when I returned to the US though: see below). These observations didn't discriminate KS1 and KS2. The symptoms varied depending only on the phone but not on SIM card. Meaning that SIM cards were okay, and some phone-internal parameter was the culprit. The Annex website said I need to go to unlockit.co.nz to set APN, but that didn't work, giving 'Profile Installation Failed / Only one APN configuration can be installed at a time' error message.
* I don't have a screen shot of the error message; the above is from Google.
[Cause] Fine, there should be an APN setting already installed. But I found nothing in settings -> general -> profile. It took me 3 days to figure out what the heck was going on. If you dial the secret code *3001#12345#* you will find more detailed technical information. Under PDP context info -> '0' tracfone.vzwentp was hiding. This ghost item was staying somewhere deep inside the iPhone, not giving way to a new APN setting. But you cannot remove this item simply by 'Reset network setting', 'Reset All setting'. Even factory reset by 'Erase all content and setting' did not resolve the problem. In fact, call / text / data all work fine immediately after factory reset. But the same error message and problem comes back after you restore iPhone. Another evidence for an issue with iPhone itself not with SIM card.
[Attempted solution] Excluding internal setting of iPhone backup during restoration process could be a solution. But unfortunately iPhone doesn't support partial or selective job; only full backup and full restoration are allowed. There are 3rd party software where you can do these at 40~50 bucks. But I didn't want to spend that much money just for one time use. Some offered trial version, all of which didn't seem reliable / trustworthy to me.
[Symptom 2] KS2 on PH2 worked fine throughout while in Korea. However it failed working when I replaced Tracfone SIM card upon arrival to the US, this time with different error message: 'Profile installation failed / The UUID for the profile "tracfone.vzwentp" is not unique'. Again, call / text works good but no cellular data. I tried a few sites such as iapnupdatevzw.tracfone.com, ireseller.tracfone.com, iphone.flexserve.net/about according to Google search result. None of them worked, with the error message being the same.
[Final solution] You can modify the backup copy, using iBackupBot. Restoring iPhone with the updated backup copy is equivalent to selective restoration. Specifically, either modify (see the comment by antonl.anton in this thread) or delete (see the comment by tonyhabayeb in this thread) SystemFiles/ManagedPreferencesDomain/mobile/com.apple.managedCarrier.plist file. In my case modification didn't work nice but deleting did. Probably iPhone creates a new plist file if it doesn't exist. Note that I used trial version of iBackupBot only. If the program stops for an error, just ignore and retry. Don't forget to make a copy of the backup just in case.
In short, the APN information Tracfone SIM card writes to your iPhone, not completely deleted when the SIM card is removed, stays like a ghost preventing installation of other APN. I chatted with both Tracfone and Annex reps but they kept telling me to do what I could easily find from Google search. Or recommended me to ask Apple(!).
I managed to find solve the problem, but only after only-God-knows-how-many tedious cycles of iPhone backup / factory reset / restoration / etc... a headache. Hope you don't.
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