I found this in the 3 PAYG SIM Only Terms and Conditions: Perhaps there is a setting in the N900 which disables 3G but leaves 2G enabled? If so, that would cause 3 to invalidate the SIM. The reason 3 does this is that they have to pay another network (Orange?) for 2G roaming, so they lose money if someone uses a handset that's only capable of 2G operation. Roger