Users often confuse "IMEI changing" with "network unlocking." Unlocking a phone to use different SIM cards is legal in many regions and can be done via Official Unlocking Services using your original IMEI.
A technician will repair a null IMEI—they will not change it to a different number. Reputable service centers refuse to alter IMEIs. Nokia X1-01 Imei Change Code
How to fix an invalid IMEI issue on your smartphone - Airtel Users often confuse "IMEI changing" with "network unlocking
If your Nokia X1-01 is locked to a specific carrier and you want to use another SIM card, search for instead. That is a different, legal process. You can request an unlock code from your carrier or a third-party service for a few dollars. Nokia X1-01 Imei Change Code