IMEI identifies the device, SIM identifies the subscriber. IMEI is used to block stolen phones, but it's also how phones are locked into specific carriers (which used to be a much bigger part of the business model, "$0 phones" and all).

Imei can be changed on some phones, and are not secure. The sim card needs to be logged to from the phone (usin PIN for uset-level login), contains non readable secrets and provides a completely separate execution environment with its own memory and cpu.

What I was saying is that it doesn't look like IMEI is essential part of the protocol and perhaps was added because of that and similar cases? Without any identifiable ID of the phone, you wouldn't need a burner phone, just a burner SIM.

SIM stands for "Subscriber Identity Module". IMEI stands for "International Mobile Equipment Identity". One identifies the account/person connecting (your plan, usage, etc.) while the other identifies the phone (and its capabilities on the network).

What I was trying to say, I don't believe IMEI would be needed for the phone to call/receive calls, and perhaps was invented after the incident.