The device IMEI is used to establish account identity for cellular service. It's intrinsic to functioning as a mobile device.

IMEI is not GAID (Google Ad ID) or IDFA (Identifier for Advertisers, for iOS devices), which can be changed. Google Android allows users to change GAID (in a rather cumbersome process), some privacy-conscious Android alternatives automate this on a regular basis AFAIU.

IMEI doesn't function like a MAC address, which can also be changed with relatively little concern (though it's helpful to present the same MAC to the same network on repeated connects, particularly if that network limits access to known MAC addresses, a ... rather weak form of security).

Moreover, IMEIs are useful in limiting the usefulness of stolen devices, as the IMEI can be added to a blocklist by carriers to prevent their use on networks. There's been (unsuccessful to date) legislation proposed in the US to ban IMEI modification entirely. In practice it is possible to change IMEIs, but that would effectively result in the device being unrecognised by the carrier, and new service under the new IMEI would have to be established. This isn't something you could do easily while continuing to use the same number (absent, say, number portability ... which would defeat much of the identity skirting), though it might fit some use cases.

IMEI is largely present only on phones with SIM or eSIM capabilities, but is independent of the SIM itself. Changing the SIM/eSIM will NOT change the IMEI.

<https://en.wikipedia.org/wiki/International_Mobile_Equipment...>

More generally: it is hard to make cellular device use private, given that effective identity leaks occur through so many channels. Location, proximity to other devices, patterns of use, patterns of contacts, billing information, associated phone numbers, other account contacts, and the like. Much as I'd prefer otherwise, a given phone probably maps pretty closely with an individual or small group (family, household, business location / work crew, etc.). That's pretty intrinsic to how the system functions.

Securing data on the device may be more tractable, but limits exist there too.