That makes no sense. How could they possibly detect that?
Well cellular telecommuncations aren’t exactly my specialty, but there has to be some sort of negotiation with the tower when connecting, right? If I had to guess it’s just “If phone doesn’t successfully negotiate to join the tower → mark this phone as incompatible with the network”
It just so happens to be mine…
A frequency band is either supported or not. If no supported bands are available, you can’t communicate at all.
Theoretically it can decide to not like what the phone is capable of; but only after having been able to actually communicate. (But since it could communicate, and it is a fairly normal/capable phone… why would it?)
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Well I’ve certainly got no good guesses for the behavior that I and other people have seen, none that explains that other people with identical hardware running the same firmware with the same configuration seem to have no problems. iirc nothing too productive came up in ofono logs before when they’ve been posted either. @robthebold 's post indicated it might have something to do with dropping the network, which is definitely a common thing here, but that wouldn’t explain why I’m able to take the sim out and put it back in, reboot the phone, etc when I have good signal.
@flypig I’m not sure of the percentages, but would it be worth adding a caveat on the T-Mobile entry for the 10ii that these problems have occurred and that T-Mobile’s official position (at least from what their engineers told me,) is that a 10ii as a device isn’t accepted/supported for BYOD on the network?
Thanks for performing the testing and digging in to this. I think it’s good to make clear anything that helps other users understand capabilities or potential difficulties. Could you propose a short and pithy way to describe the phenomenon? If it’s hard to encapsulate in a sentence, then perhaps it would be better to add a link to your post instead?
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“Some users have experienced being kicked off of the network with a ‘Denied’ status message which can only be circumvented by putting the SIM into a supported phone and letting it connect to T-Mobile. T-Mobile claims that this is because the 10ii does not support their network.”
Just the first sentence would do, but the 2nd seems like a fairly sensible warning.
Just to make things a little murkier, if you put the IMEI of a (or at least, my) Xperia 10 II in T-Mobile US compatibility checker, it says it’s “OK”. 
And to muddle things even a bit further, over New Year’s Holiday I drove across the great state of Missouri (or is it Misery?) and went through several T-Mobile dead zones. One right under a T-Mobile billboard bragging about their network coverage 
But in those cases, I got re-connected when signal improved. This seems in odd contrast to losing a connection because of my position inside a structure in an urban area with good overall coverage otherwise – that scenario always led to a “Denied” status even after getting out from under the blocking material.
I have an answer to the first part, at least. T-Mobile doesn’t know that the 10 ii exists. If you look at your t-mobile devices you’ll see that as far as they know you have an Xperia Z1S.
Thanks for the wording @tekktonic, that’s very helpful. I’ve added a sentence to the table, including a link to your message to explain the full situation.
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Rather annoying update: since my testing a few days ago I seem to be getting Denied any time I put the SIM into my 10 ii 
I meant to try and induce a disconnect by going to a place I know won’t have signal and logging, but I’m not sure if I can do that.
Are you still “Denied”? Does the SIM-switcheroo trick help get it un-denied in your case?
Yes. Leaving it in an official t-mobile phone even for as long as 24 hours and switching isn’t working atm. I’m hoping that the denial is just being cached somewhere for a few days rather than permanently flagging the IMEI or similar (I guess I could test that by using SIM 2, but I use my SD card so that wouldn’t exactly be a permanent solution.)
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maybe someone can shoot down this possibility, but a variant of my idea before: could it be locking the devices out because they try to connect on 2g or 3g? That’d explain why a seriously rural area wouldn’t knock it out: it has no signal at all. It also might explain the somewhat random nature that we see, if the phone just happens to try and connect via 3g first. Some rule like “If phone isn’t from us and it connects over 3g, it’s misconfigured and should be locked out of the network”
Is there some secret way to tell Sailfish to not even attempt to fall back and only use 4g?
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That sounds unlikely. It is reasonable for any device to do that - so they would have to except approved devices then. And what do they gain except a lot of faff? Isn’t 3G off now anyway? It would be very costly to keep running just to mess with people.
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In the US, the network operators seem to only want to grant their desired devices access to their network.
In the Volla forum there is an interesting thread on the subject where customers try to access the networks with their Volla22.
“…for the Volla22 it maybe an IMEI and frequency band issue too”
In this Post there is an interesting video concerning “Mobile Carrier IMEI Checks”
With the result that they say your phone is not compatible.
I just hopped to t-mobile recently, but my understanding is that they’ve shut down the 3G network but are keeping the 2G network online for another year for legacy devices. In the specific case I proposed I’m guessing the trigger would be a not-registered-to-tmobile device connecting to the 2G network. I seem to recall support calls months ago where T-Mobile told me that my device had been flagged as being a 2G device, though that was over way back in June I think so my memory may be fuzzy.
They might say that… but presumably they really mean non-VoLTE.
So failure to register for VoLTE might cause this… but trying an available older technology should not.
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Maybe? I’m not sure why 4G registration would randomly fail though. I could plausibly see 2g being treated as a “legacy” thing and not letting already-known IMEIs join in theory. I believe they did something like that with 3g? Like, no new 3G registrations for a year after shut down. This wouldn’t be too different from that in theory.
I think weak connection and legacy speed bands cause this…?
No. And there is no such thing as “legacy speed bands”.
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