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These codes open either the IMEI editor directly, or the engineering menu where it can be found (like "Para Set" - "Update IMEI Number" on Mocor-based phones). If you view this page from a feature phone itself, you can dial the code directly via its WTAI link or save it to your phonebook. In some cases, you'll be required to press the Call button after entering the code.
P.S. All the codes were tested on various existing non-Nokia feature phones manufactured in 2016 or later. If you can't find the one working on your phone, consider making a firmware dump depending on your chipset (for instance, with or ) and looking for the appropriate one inside.