For a long time now we've had some customers with iPhones receive a SIM and activate it from their Charge account, only to report some very strange behavior when they pop the SIM into their device.

The status bar will show "Sprint LTE" for somewhere between two and five seconds, and then the connection will just drop. The status bar will only show "No Service" no matter how many reboots and re-activations and factory resets are performed.

From experience (and our own internal testing) we've learned that this happens almost exclusively on iPhones purchased outside the United States. That's what led us to put a warning up on our device support page saying that international iPhones aren't compatible with Charge.

Until recently we didn't really know why. Sprint wasn't able to tell us much, other than confirming that they see the same behavior from internationally-purchased iPhones and suggesting it's a problem on Apple's end related to something called an "activation policy."

Not satisfied with that answer, we decided to investigate on our own.

On a recent trip to London, our CEO Andrew stopped by the beautiful Covent Garden Apple Store and purchased an unlocked SIM-free iPhone 6, model A1586. According to Apple's LTE compatibility page this model is compatible with Sprint's network in the United States. And indeed, we have customers using that exact model on Charge already.

Andrew brought the iPhone back to San Francisco without opening the box. We activated it on Charge with a Charge/Sprint SIM, just like we would with any other iPhone. And immediately after booting it up, we saw "Sprint LTE" for five seconds, followed by the dreaded "No Service" exactly as our customers had described.

Having replicated the issue, we set out to dig deeper. Could this refusal to maintain a network connection really be a problem on Apple's end? Why would an unlocked SIM-free iPhone bought outside the United States behave differently than an iPhone with an identical model number bought here in the US?

We called Apple support. At first they told us that we should try a backup and restore of the iPhone through iTunes. Even though we pointed out that the iPhone exhibited this strange behavior upon its first boot ever, the agent insisted. So we backed up and restored the iPhone. Naturally this did not result in any change.

So we called back. We were told that the iPhone was probably locked. After pointing out that the iPhone was bought unlocked and SIM-free in the UK and meeting continued resistance, we asked for an escalation to a support manager.

Things moved a little smoother at that point. It was suggested that we manually force a network settings reset on the iPhone. We had already tried this, but we performed the reset again and of course saw no change. We asked whether this might be related to an "activation policy" issue, but our support manager didn't know.

We got off the phone with an agreement that the support manager would investigate internally to see if he could find any other steps towards resolution, and if not would initiate an engineering request to investigate.

We went on to do some more digging while we waited and came across this HowardForums post which suggested a SIM-free iPhone 6 bought outside the US might also have trouble connecting on Verizon's network. This was an important clue, and suggested that the problem might be due to differences in the way iPhones connect to CDMA and GSM networks.

We sent the post to Apple support via email and were told that an engineering request had been opened. After four days of waiting, we got word that Apple engineering had responded to our request and perhaps had resolved the issue.

Apple engineering had altered the activation policy associated with our UK iPhone and enabled it for "multi-mode" network access. On the phone with our Apple support manager, he said that the previous activation policy was incompatible with CDMA networks due to restrictions related to sharing the iPhone's MEID with the network provider. CDMA networks like Sprint's require the MEID as an identifier during network authentication. So any iPhone with an MEID-restricting activation policy will associate with a CDMA network, but not authenticate. Hence the few seconds of "Sprint LTE" that appear, followed by "no Service" forever.

Our support manager said one more backup and restore of the iPhone would give it the new activation policy and that might just solve the connectivity problem. We performed the backup and restore from iTunes one more time and booted up the iPhone. This time, we saw "Sprint LTE" in the status bar and it held firm. We opened Safari and downloaded a web page. Our long battle was over. We had connectivity!

We don't know why Apple is selling non-US iPhones with this CDMA-incompatible activation policy in place, but whatever the reason we feel confident that we know the source of this very unfortunate compatibility issue.

We hope that by posting our findings and a path to resolution we'll give our customers and anyone else suffering from this problem at least a little hope that with enough effort they can use their foreign-bought iPhones in the United States on any CDMA or GSM network.