de0u
Is it possible to cite official statements from Korean carriers about this? What happens when tourists visit?
They engage in wordplay, so even if there is an official statements, it is meaningless. For overseas devices, the position is that since the carrier does not sell the device, there is no reason to provide official statements in the first place.
Users cannot do anything other than request their carrier to redefine the OMD code. (This feature exists not to accommodate foreign devices, but rather to impose restrictions by using a separate OMD code. but, Starting with 5G, the OMD codes that previously existed separately for each device have been consolidated into a single code, making them less strict.)
That sounds unfortunate, but I am skeptical that the GrapheneOS project would maintain code to spoof SIP user-agent registration -- at least not without solid documentation of the problem.
I certainly agree that, along with issues related to microG, they would be reluctant to insert code that induces spoofing behavior. However, since the communication protocol is proprietary, it is also expected that this cannot be resolved with an open concept.