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[inquisitor]

I've just found out that the iPhone - at least unless jailbroken - does not provide access to the SIM Toolkit (STK), but on any other phone you should find this item somewhere in the menu (old Nokias just call it "Services") or among the apps (Android). Once you start the STK, you will see a first item called "toggle". After selecting "toggle" in the next menu level you may chose between manual and automatic IMSI selection. There you would need to select manual IMSI selection, which will reveal a list of activate IMSIs for your SIM card. In your case it should list "Global", "UK" and "Poland". Here you would select "Global" and then perform a manual network search after which you would try to register on each of the listed networks. If your registration fails for a certain network retry it four times to rule out that it's just the steering mechanism which prevents you from registering.