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After the PUK code is entered, the PIN must be reset. If the wrong PUK is entered ten times in a row, the SIM card will become permanently blocked and unrecoverable, requiring a replacement. Mobile phone users are therefore advised by most providers to keep their PUK written down in a safe place separate from the device.
ETSI and 3rd Generation Partnership Project (3GPP) standards, such as GSM and LTE, define supplementary service codes that make it possible to query and set certain service parameters (e.g., call forwarding) directly from mobile devices.
This is in contrast to a phone (retrospectively called SIM-free or unlocked) that does not impose any SIM restrictions. Generally phones can be locked to accept only SIM cards with certain International Mobile Subscriber Identities (IMSIs); IMSIs may be restricted by: Mobile country code (MCC; e.g., will only work with SIM issued in one country)
Mobile phones use geographic area codes. Exchanges may service on mobile devices; local numbers are portable between wired and wireless carriers. While area code 600 has been established as a non-geographic code that can be used by mobile phones, the only significant mobile usage has been for satellite phone service in remote regions.
This list contains the mobile country codes and mobile network codes for networks with country codes between 300 and 399, inclusively – a region that covers North America and the Caribbean. Guam and the Northern Mariana Islands are included in this region as parts of the United States.
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The user submits a code corresponding to the service available. USSD can be used to provide independent calling services such as a callback service (to reduce phone charges while roaming), enhance mobile marketing capabilities or interactive data services. USSD is commonly used by prepaid GSM cellular phones to query the available balance.
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