Editing SIM lock (section)

{{Short description|Restriction built into a mobile phone}}

A '''SIM lock''', '''simlock''', '''network lock''', '''carrier lock''' or ('''master''') '''subsidy lock''' is a technical restriction built into [[GSM]] and [[CDMA]]<ref name=Price>{{cite web | url=http://www.phonenews.com/in-depth-why-iphone-4s-cant-use-cdma-on-gsm-networks-17843/ | title=In-Depth: Why iPhone 4S Can't use CDMA on GSM Networks | last1=Price | first1=Christopher | date=October 8, 2011 | website=PhoneNews.com | publisher=[[Mobile Media Ventures]] | access-date=September 24, 2016 | quote="CDMA ... us [es] an antiquated Master Subsidy Lock code system."}}</ref> 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.

Generally phones can be locked to accept only [[Subscriber Identity Module|SIM]] cards with certain [[International Mobile Subscriber Identity|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 network code (MNC; e.g., [[AT&T Mobility]], [[T-Mobile US|T-Mobile]], [[Vodafone]], [[Bell Mobility]] etc.)
* Mobile subscriber identification number (MSIN; i.e., only one SIM can be used with the phone)

Additionally, some phones, especially [[Nokia]] phones, are locked by group IDs (GIDs), restricting them to a single [[Mobile virtual network operator]] (MVNO) of a certain operator.

Most mobile phones can be unlocked to work with any [[GSM]] network provider, but the phone may still display the original branding and may not support features of the new carrier. Besides the locking, phones may also have [[firmware]] installed on them which is specific to the network provider. For example, a [[Vodafone]] or [[Telstra]] branded phone in [[Australia]] will display the relevant logo and may only support features provided by that network (e.g. Vodafone Live!). This firmware is installed by the service provider and is separate from the locking mechanism. Most phones can be ''unbranded'' by [[Flashing (technology)|reflashing]] a different firmware version, a procedure recommended for advanced users only. The reason many network providers SIM lock their phones is that they offer phones at a discount to customers in exchange for a [[contract]] to pay for the use of the network for a specified time period, usually between one and three years. This [[business model]] allows the company to recoup the cost of the phone over the life of the contract. Such discounts are worth up to several hundred [[United States dollar|US dollars]]. If the phones were not locked, users might sign a contract with one company, get the discounted phone, then stop paying the monthly bill (thus breaking the contract) and start using the phone on another network or even sell the phone for a profit.<ref>{{cite web|url=https://www.gazelle.com/thehorn/2014/01/29/what-is-a-locked-phone-and-why-do-carriers-lock-them/|title=What is a Locked Phone and Why do Carriers Lock Them?|date=29 January 2014}}</ref> SIM locking curbs this by prohibiting change of network (using a new SIM). 
 
In some countries, SIM locking is very common if subsidized phones are sold with prepaid contracts. It is important to note, however, that the technology associated with the phone must be compatible with the technology being used by the network carrier. A GSM cell phone will only work with a GSM carrier and will not work on a CDMA network provider. Likewise, a CDMA cell phone will only work with a CDMA carrier and will not work on a GSM network provider.<ref>{{cite web|url=https://blog.unlockbase.com/10-most-frequently-asked-questions-pertaining-the-fcc-guidelines-mobile-device-unlocking/|title=10 Most Frequently Asked Questions Answered Pertaining the FCC Guidelines on Mobile Device Unlocking|date=27 February 2015}}</ref><ref>[https://www.unlockninja.com/blog/how-to-check-the-network-product-code-and-country-of-your-phone/ Ways to check the Network, product code and country of your Phone] Guidelines to find about your network provider</ref> Note that newer (2013+) high end mobile phones are capable of supporting both CDMA and GSM technologies, allowing customers to use their mobile devices on any network. Examples of these mobile devices are the Apple [[iPhone 5c]], 6 and newer, [[Motorola g4|Motorola's G4]], G5, X Pure, [[Samsung Galaxy S6|Samsung's Galaxy S6]], [[Samsung Galaxy S7|S7]], [[Samsung Galaxy S8|S8]] smart phones, mostly phones based on a [[Qualcomm Snapdragon]] chipset or radio.

In some jurisdictions, such as [[Canada]],<ref name="crtc-simlockban" /> [[Chile]],<ref name="Chile-ban" /> [[China]], [[Israel]],<ref name="Israel-ban" /> [[Singapore]],<ref name="Singapore-ban">{{cite press release|title=TAS Fines M1 For Unauthorised Frequency Transmission And Issues Warning Over Sale Of SIM-Locked Cellular Phones|date=1997-09-08|publisher=Infocomm Development Authority of Singapore|url=http://www.ida.gov.sg/News%20and%20Events/20050721123258.aspx?getPagetype=20|archive-url=https://web.archive.org/web/20070812084537/http://www.ida.gov.sg/News%20and%20Events/20050721123258.aspx?getPagetype=20|archive-date=2007-08-12}}</ref> and the [[United States]]<ref>title=S.517 - [[Unlocking Consumer Choice and Wireless Competition Act]]|access-date=20 January 2025|publisher=[[congress.gov]]|url=https://www.congress.gov/bill/113th-congress/senate-bill/517</ref> it is illegal for providers to sell SIM locked devices. In other countries, carriers may not be required to unlock devices or may require the consumer to pay a fee for unlocking.

Unlocking the phone, however, is almost universally legal.<ref>{{cite web|url=http://www.simonlymetinternet.nl/is-iphone-unlocking-illegal-en/|title=Is iPhone unlocking illegal?|publisher=SimonlymetInternet|access-date=23 September 2012}}</ref> Additionally, it is often legal for carriers to force SIM locks for certain amounts of time, varying by region.