Fairphone 6 Unknown Caller ID Due to Modem IMS Mismatch

The primary cause of this issue is a mismatch between the device's modem firmware and the carrier's (A1 Croatia) IMS profile, often manifesting as an 'Error 38' or similar low-level authentication failure during the logical channel handshake. To confirm this cause, check the system logs or debug information for IMS registration errors when the 4G Calling feature is active. Additionally, verify if the issue persists across different SIM profiles, such as testing a prepaid versus a contract SIM, as the IMS authentication handshake may succeed or fail based on the specific network provisioning associated with the SIM card.

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