It may have been bought within the brief window that Vodafone locked their handsets.
Call Vodafone customer services with the handset IMEI number and they may be able to help you.
Otherwise it shouldn't be network locked and the lock code it asks for is related to the handsets security settings which may be set to "Ask for PIN if SIM changed"
As above the standard security code is 12345, if this doesn't work and you've forgotten the security code then you'll need to send it to Nokia Care to have it reset.
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