Dean,
Only those carriers that have branded an eligible model will even see the RTM.
Straight Talk can't release what they will never be given.
When a phone is branded by a carrier, that carrier gets an RTM (Release to Manufacturer) to test and pass for push or decline. Same goes for OEMs like HTC. So if you have an HTC One (M8) for Windows you are under a double whammy. HTC has to pass the upgrade and ATT has to pass the upgrade before it gets pushed.
If you SIM unlock your ATT Branded HTC One (M8) for Windows and take it to T-Mobile... Your upgrade comes courtesy of, or held hostage by, HTC and ATT - no matter that T-Mo also had a branded One (M8).
The SIM that is in the phone has zero to do with where your upgrade comes from.
The upgrade push targets devices by class based on the hardware identifiers in the database. Once an ATT device, always an ATT device so far as the upgrade checking your phone is concerned.
You can fly an ATT One (M8) to Italy and Italy has nothing to do with the upgrade. You can shove a TIM Italy SIM in it. TIM has nothing to do with the upgrade...
Straight Talk is very much a moot point. They are
NOT in the upgrade path in any way shape or form.
You can take an ATT One (M8) to China. Get it SIM unlocked from ATT. Have no SIM in it. The moment HTC and ATT approve the upgrade, you can put the device on WiFi and get the upgrade. No SIM required.
If your device is not carrier branded (and you seem unwilling to say), you can use Upgrade Advisor right now and get W10M official.
If your device is carrier branded, you are waiting for that original carrier to approve the official upgrade no matter what SIM is in the device.
If your device is carrier branded and you are tired of waiting for official W10M, you can install Insider App today and get the same build as official by selecting the Release Ring and upgrading. Then check updates and bump up from the entry version to 10586.420 the same as everyone else has...
In the end, you have way more to do with the upgrade than Straight Talk ever could. To use a common phrase: "You're barking up the wrong tree."
Good Luck.
[NOTE]If your phone is on the eligible list and not waiting for carrier approval (branded phones) use
Upgrade Advisor to get the Official release (RTM).
If you use Insider on the Release Ring, the same version is fetched as the recent public Upgrade received by these phones (10.0.10586.xxx): Phones Eligible for Windows 10 Mobile Official.
If your phone is not on the supported list, then 10586.xxx (Threshold) is the end of the line. You may as well use Insider and select Production to un-enroll your phone from Insider, but at least you have W10M. WaaS (Windows as a Service) will deliver future updates within Threshold to you without any Insider access required.
If your phone is on the eligible list and you go Insider and stay on the Release ring you get the next public version early as Release ring in effect is the RTM. This will deliver the most stable Redstone builds to you earlier than OTA push with minimal risk.
Check out this thread: Windows Insider Testing and You [Recommended Guidelines]
For right now, Windows Device Recovery Tool is only flashing 8.1 to models that did not launch with 10 so it is the way to revert if you don't care for W10M: Windows Device Recovery Tool FAQ[/NOTE]