[123919239]

You cannot accept unknown SIM cards due to mutual authentication -- the provider must know the shared secret that is burned into the SIM.

You can achieve network separation APNs, so there is no need for such a hack.

The question I have in mind is: Does your office advertise it's own PLMN with WiFi-like equipment, or do you outsource your office wireless network to an established operator which handles authentication and provides a virtual, separate network?

[Jhsto]

Oh nice to know the thing about the SIM cards!

Regarding your question, I recall reading a paper which compared both cases: the office owning the whole infrastructure, and one in which it is rented/outsourced to an established MNO. While the paper suggested that both use-cases are plausible, in practice your preference is likely to vary depending on local regulation and use-case (e.g. latency requirements etc.).