Unlocked from all networks: how to check and ensure that the device works with any SIM card

One of the most common questions in the secondary market for smartphones and other mobile devices is whether the device is “unlocked” (network unlocked / SIM free). This is very important for buyers who are choosing a used phone, planning a trip abroad, or simply want to change their service provider. Although factory unlocked devices are increasingly sold in Europe, the practice of locking is still widely applied in some regions (e.g., the USA) or when purchasing devices on installment with a carrier contract.

In this article, we will explain in detail what network locking is from a technical perspective, how to reliably check the device’s status, and what to do if your phone is tied to a specific carrier.

1. The technical anatomy of network locking (Network / SIM Lock)

Network locking is not just a simple software setting that a user could easily change from the smartphone menu. It is a restriction integrated at the operating system and baseband (baseband processor) level.

When a device is locked, a specific combination of MCC (Mobile Country Code) and MNC (Mobile Network Code) codes is written into the firmware of its baseband processor. These codes identify countries around the world and specific carriers. When a SIM card is inserted, the phone reads the IMSI (International Mobile Subscriber Identity) number contained within it, which encodes the MCC and MNC at the beginning.

If the SIM card codes do not match the codes in the phone’s firmware “whitelist”, the baseband processor rejects the connection request and displays a message on the screen, such as: “SIM Not Supported” or “Network locked”. Carriers use this practice to protect their investments when the phone is sold subsidized (at a lower price) in exchange for a long-term service contract.

2. Factory Unlocked vs Software Unlocked

When looking for an unlocked device, it is very important to understand the difference between factory unlocking and modified unlocking.

Type	Method	Reliability
Factory Unlocked	Factory or official carrier unlocking via IMEI servers.	Permanent. You can confidently update the OS and restore factory settings.
Software Unlocked	Software hacking (Bypass), special chips (R-SIM).	Low. After an operating system update, the device may lock again.

3. Verification methods: How to find out if the device will work with all SIMs?

To avoid unpleasant surprises, it is recommended to perform a multi-step check.

A. Physical “Dual-SIM” or different carrier test

The simplest and most reliable method. Turn off the device, insert a SIM card from another local or foreign carrier, and see if the phone finds the network and allows calling. If prompted to enter a “Network Unlock PIN” – the device is locked.

B. System settings check (iOS and Android)

Apple iPhone (iOS 14+): Go to Settings -> General -> About. In the “Carrier Lock” section, it should say “No SIM restrictions”.
Android devices: Settings -> Network & Internet -> Mobile Network -> Advanced. In some models (e.g., US market), there is an app called “Device Unlock” indicating “Permanently Unlocked” status.

C. IMEI database check (IMEI Checkers)

Dial *#06# on the phone and check the obtained 15-digit IMEI code in special databases (e.g., SickW or IMEI.info). This will provide official information about the carrier lock status.

4. What to do if the phone is locked?

Official route

Contact the original carrier. If the contract has expired and there are no debts, the carrier must unlock the device for free. This is the safest way.

Third-party services

Remote IMEI Unlock services can help regain freedom for a fee. Choose only reliable providers (e.g., Trustpilot rated) to avoid scams.

5. The invisible barrier: Frequency bands (LTE / 5G Bands)

A device may be “Network Unlocked”, but still not function properly due to technical frequency differences between regions (North America vs Europe).

“For example, a phone designed for the US market may not support the European Band 20 (800 MHz) band, resulting in poor or no connectivity in rural areas.”

6. eSIM technology and the future of locking

The transition to eSIM fundamentally changes the locking process. Carriers lock the EID (Embedded Identity Document) number itself. Software “hacking” (like with R-SIM) is nearly impossible in the case of eSIM, making the official “Factory Unlocked” status even more important.

Conclusions

Always check the device’s unlocking status before purchasing or traveling. It is best to choose factory unlocked (Factory Unlocked) models and check frequency compatibility to avoid connectivity issues.

Sources and recommended literature:

GSMA (2023). IMEI Security and Network Locking Guidelines. Official technical specification.
Mulliner, C., et al. (2012). SMS of death: from analyzing to attacking mobile phones on a large scale. Baseband processor analysis.
Kushwaha, A., & Gupta, S. (2021). An empirical study on the secondary smartphone market: Information asymmetry and pricing.
Apple Inc. (2024). How to unlock your iPhone for use with a different carrier (Support document HT201328).