iPhone was not off with new pixel
iPhone was not turned off when carrier switch number to pixel phone which had existing SIM card for different number can’t dereigister sim in old iphone now
iPhone SE, iOS 14
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Where is the SIM with the Old Number, If it is not with you or you have given it back to the Carrier Service Provider, Then don't bother about it? Then de-registeration is not required and also not possible.
If you have it with you and want to use it with an Android device then put the SIM on any Phone (Apple or Android) and follow the GIF
Insert the same SIM into an Android or an iPhone. You may deregister iMessage online
Watch the GIF below. Here I have used another iPhone for the demo but for you, it may your choice. Android or iPhone device.