Here's why some OnePlus 5s rebooted when calling 911, and how OnePlus fixed it
This is what you'd call a 'showstopper' bug ... if you actually find it.
OnePlus has offered up more information on the worrisome bug found by some OnePlus 5 owners that would reboot the phone when trying to place an emergency call. After some investigation, it turns out a confluence of issues combined to cause phones to sometimes reboot when calling over VoLTE to emergency numbers that supported OTDOA positioning information.
OTDOA, as you likely aren't aware, is a system that uses LTE signals to determine a phone's location by pinging multiple cell towers and measuring how long it takes each signal to reach the tower. If the emergency number you're calling supports the system, the phone will report the its approximate location using OTDOA. As you can imagine, that kind of location information is extremely useful in an emergency situation.
An immensely helpful OnePlus 5 owner helped the company quickly pinpoint and fix the problem.
As we found out in the days following the initial reports of 911 reboots, the issue wasn't happening on all OnePlus 5 models or in all countries. This is pretty simply explained by the fact that not everyone is making calls with VoLTE (the OnePlus 5 only supports it for certain networks), and not all emergency numbers necessarily take OTDOA information even if you are. It's still a really bad bug no matter what, but OnePlus was thankfully able to act quickly to fix it. After contacting the user who initially reported the issue, OnePlus was actually able to obtain device logs that pinpointed the core problem.
It turns out the root of the whole problem wasn't just the OnePlus firmware, but rather how it interacted with the actual cellular modem in the phone that's part of the larger Qualcomm system-on-a-chip powering the OnePlus 5. The process of obtaining and sending OTDOA data caused a memory issue in the modem that eventually caused the reboot. Without the device log information from someone who saw the issue, it would've been notably harder to determine the real problem — and, indeed, the bug did make it through software testing all the way to production. OnePlus and Qualcomm are confident now, though, that this bug has been fixed via the OTA update that rolled out on July 21.
This isn't the sort of bug we ever want to see make it into any phone, but it's great to see OnePlus — with the help of the users who first spotted it — fix the issue in just a few days and help out the people who may call emergency services on their OnePlus 5 in the future.
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