Understanding why users attempt this process helps contextualize the technical hurdles involved:
In many jurisdictions (including the US, UK, and European Union), changing a device's IMEI to mask its identity, bypass carrier blacklists, or clone another device is illegal under laws such as the Wireless Telegraphy Act or the Mobile Telephones (Re-programming) Act.
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How to Change the IMEI on a Fibocom L850-GL LTE Module: A Technical Guide