The phone is probably SIM locked to a particular network. Is it a pay monthly or Pay as you go PAYG deal? Pay monthly or contract tariff phones are usually unlocked to any SIM where as PAYG handsets are locked to one network.
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Or the SIM has not been activated by it's network or deactivated because it hasn't been loaded for a long period of time?
The phone is probably SIM locked to a particular network. Is it a pay monthly or Pay as you go PAYG deal? Pay monthly or contract tariff phones are usually unlocked to any SIM where as PAYG handsets are locked to one network.