[Scoundreller]

I also kinda figure there’s some magic running to “stick” to your home network where available/visible because of international border areas and people historically getting regularly upset about being roaming charges despite never leaving their home country.

[lxgr]

SIMs can define both their home network (both implicitly since the IMSI starts with MCC/MNC of the issuer/home network, and explicitly in the form of a list of "equivalent networks", which is useful for MVNOs with their own MNC that don't want the "roaming" icon to show up) and a ranked list of preferred roaming networks. The phone should usually define those.

Of course, in some situations you might only get signal from across the border, and then none of these mechanisms can help.

[Scoundreller]

I’d add in some high buildings in Toronto, if I did a network scan with a foreign SIM, I could see some US networks from over the lake, but with a Rogers SIM, they would not be visible in the scan.