The IMEI is an identifier that identify the mobile phone and it's on the network.
With it, it's also possible to get the brand and model of the mobile phone being used, and some operator did go as far as showing the brand and model of the phone on the customer web interface.
Legal aspects of changing the IMEI¶
We would need to do some more legal research on that to know in which conditions it's legal or not.
Though using random MAC address for privacy is now done by many OS and distributions.
Operator blocking non approved mobile phones brands or models¶
In the European Union, the net neutrality regulation makes sure that user "can use the equipment of their choices" (Tim Wu (2003). "Network Neutrality, Broadband Discrimination" (PDF). Journal on Telecom and High Tech Law. page 169: "rule barring the Bells from preventing attachment of equipment unless harm to the network could be shown") to access the network.
So while there is some IMEI blocking in practice, as for instance the IMEI 00000000[...] is blocked in many networks, it's not legal to block devices based on their IMEI if that IMEI is valid and that the device is not stolen, etc.
For instance Operators aren't supposed to block older devices, devices from less known brands, devices from a specific brand, devices not tested or approved by the operator, etc.
We don't have any report of less common mobile phones like the Openmoko, or the GTA04 being blocked.
1 This has been observed with OsmocomBB, by forgetting to add the IMEI inside the configuration file and trying to register, which didn't work. Once the proper IMEI (which is written on the back of the phone below the battery) was added to the configuration file, registering to the network worked.
There are some IMEI blocking going on too, but the articles tend to point out that valid IMEI aren't blocked.
This started recently (2018 and 2019), and we don't have any report of less common mobile phones like the Openmoko or the GTA04 being blocked.
Nevertheless, there is a website that can use to check if a given IMEI is blocked or not, but it only works from Brazilian IP addresses (so it's less practical if you intend to travel there): https://consultaaparelhoimpedido.com.br/public-web/welcomeThere are some articles on the topic:
If I understood correctly, the articles says that if the IMEI is in the GSMA database it should be ok, but I don't speak Portugese.