<P> A SIM lock, simlock, network lock, carrier lock or (master) subsidy lock is a technical restriction built into GSM and CDMA mobile phones by mobile phone manufacturers for use by service providers to restrict the use of these phones to specific countries and / or networks . This is in contrast to a phone (retrospectively called SIM - free or unlocked) that does not impose any SIM restrictions . </P> <P> Generally, phones can be locked to accept only SIM cards with certain International Mobile Subscriber Identities (IMSIs); IMSIs may be restricted by: </P>

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