Can IMEI number of a device really be changed I bought a slightly used android phone(Asus Zenfone 2) recently.Now what i noticed was that the imei number that my phone was showing to me when i dialed *#06# was not the same as that on the box in fact the phone's original imei was swapped with that of some huawei device(Y6 PRO), that i came to know when i checked the imei number on some online imei device identification service (www.imei.info).Now i rooted my device and changed the imei number to the one on the box(used an app that required root privileges) and i did verify that the imei number is changed back to the original imei(Again by dialing *#06#).I made sure that the imei identifies my device make/model as (ASUS Z00AD) using (www.imei.info).Now what seemed intriguing to me was that when i called my carrier for some help regarding some issue that i faced, even though i had changed the imei number back to original one that identifies my device as ASUS ZOOAD they still recognize my device as HUAWEI Y6 PRO.Which arouses my suspicions that the imei didn't actually change and it not possible/extremely difficult to change your device imei number. Although the tools out there claim to change your imei number but what i think all they do is just change them on your phone's display not internally.Or maybe the carrier has some other unique ID through which he could identify my phone make/model.

What do you people think what did actually happen here ?

Please. Enlighten me!

Note that i am a Computer Science student , so i am just curious to know these things.I have no criminal intentions. Just curious!

UPDATE: I have also noticed that when you restore your phone to factory settings or flash a custom or a stock rom you restore the original imei.Does that mean that imei changing is not possible/very difficult? or it is stored on some kind of EEPROM from where the wireless carriers directly get the legitimate imei value Or is it some kind of trade secret to mobile manufacturers ?
IMEI is an baseband identifier so it's stored on non-writable memory. There are two IMEIs actually - display and hardware. It's possible to change the display IMEI on rooted devices, but no way to write it down to hardware. Call your carrier for resetting your SIM card and also try to flash stock firmware using factory tools (Qualcomm QFIL in your case)