[inquisitor]

I don't think IMSI selection is based on trial and error. While or before your phone tries to register on a certain network it must somehow submit the network's MCC/MNC-tuple to the SIM card because all the authentication stuff happens in the SIM card. That's where the STK application running on the SIM card comes in and decides to switch IMSIs if the requested MCC is from a country where another IMSI should be used. Of course the selection will be somehow unpredictable when you are in an border area where there are networks from multiple countries available.