Roaming when not roaming? (Foreign network while not abroad.)

This is a phenomenon I encounter more and more, especially while in areas like large company office buildings or plants.

I have a dual-SIM phone, with SIM1 set to Voice and SIM2 set to Data.

My Provider in Austria is Drei, which means my home network is MCC 232 (Austria) with network codes 05 and 10. The Service Provider name usually shows up as “3 AT”.

Now, at certain places, far from any borders, the phone shows the SIMs as Roaming.

When I check (using e.g. the app “NetMon VoLTE”) I see that we are connected to another network, with MNC 03 (T-Mobile) but the Service Provider is still given/shown as “3 AT”.

Also, when switching Network selection to manual, I can select my home network 05 usually and the Roaming goes away.

My suspicion is, as this usually happens when working at large organisations, that (in the example above T-Mobile) are providing the actual network service, but also for their competitors, i.e. pretend that they are other providers.
This way the companies their towers are at do not need to have multiple providers on their premises.

Does that make sense?

(Note: It is possible that this is totally normal behaviour and I actually do not have coverage from my native network for a time when this appears. I don’t monitor this constantly. Still, even if that is so, it seems switching back when overage gets better is not working or not working fast enough.)


Now for the interesting part:

Is there any way I can avoid this roaming-when-not-roaming and fix my selected network to MCC05 or 10 - WITHOUT disallowing Roaming generally?

On the one hand this should be a good thing for the user, because you get better coverage. On the other hand it points out to have “competing” mobile infrastructure. Imagine having competing road services…

I also noticed somehing like that. Yesterday I travelled by train in the middle of Austria (so no border to another country anywhere near) and suddenly, my Xperia 10 III reported my connection as “roaming”. My provider is HoT (using the Magenta/T-Mobile infrastructure). When I checked with NetMon VoLTE, it showed MCC 232 and MNC 03 which is the normal one it uses (also when not in roaming). Very strange.
What might also be interesting is, that this happened to me when my network coverage was apparently weak, because I was on hspa (3G or 3.5G?)

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