Actually it is a bit more complex than that.
Authorization to the network is done through the Imsi number. That’s how texts or calls can be redirected through the right full owner. This is a number generated from the interaction between the SIM card and the first tower responding to the « hello » of the SIM card.
This number can be intercepted and that’s how we are tapping into phones communications.
Then there is the pairing of the SIM card number and imei number. That’s why you can have on most telco 2 phones having exactly the same Imei number and not being rejected because they have different sim number.
Which is also usually a good way to confuse security services about it because for those who don’t have any general knowledge, they will be a bit lost.
Anyway you can change your imei number on a Qualcomm phone too. There are GitHub repository about it on how to do that if you look it up a little.
And when it comes to change the imei number of your mediatek phone, all the ressources are readily available here on the discourse about how to upgrade manually your phone.
If you do that, don’t forget to use new imei and not imei reported stolen. You have websites for that, where you need to pay.
I suggest you would generate imei on one website. And check it up on another one, a paid website this time which will check the different database accross the world to see if the imei has been stolen or not. Why it is important ? Because telco will block the phone access if they find an occurrence of this imei in the database.
Anyway, changing the imei of your phone is not illegal. Most country will not have a problem even to accept a phone without an imei. to them what is important is the generation of the imsi number.
And since the documentation is readily available, as it should, whether it is to change on Qualcomm or mediatek phone, it should not be a taboo.