I'm pretty sure a SIM is hard coded with an IMSI which is associated with a specific country and network. Therefore you can't use any SIM on any network as it's the IMSI which is used to register your phone on a network. I'd guess it's for this reason that when you port your number to another network you have to have a new SIM from the recipient network. I'd imagine the POS only need the last 9 digits in order to uniquely identify a SIM that's been supplied by that network.
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Re: Red Herring
I'm pretty sure a SIM is hard coded with an IMSI which is associated with a specific country and network. Therefore you can't use any SIM on any network as it's the IMSI which is used to register your phone on a network. I'd guess it's for this reason that when you port your number to another network you have to have a new SIM from the recipient network. I'd imagine the POS only need the last 9 digits in order to uniquely identify a SIM that's been supplied by that network.