I recently ran into the Unknown baseband problem on my Samsung Android(G900I). It always happended. I couldn't call or message and the phone baseband was "unknown" (the modem was failing to initialize). The phone randomly rebooted when the screen was off, and shutdown didn't work (it will take a very long time and would reboot.)
My Tries
1. I didn't have an EFS/modemst1/modemst2 backup. If I'd had those backups I would have saved a bunch of time! I did a new backup anyway just to be safe.
Restoring my new backup did nothing (apparently this works in some cases).
Restoring my new backup did nothing (apparently this works in some cases).
2. I was already on stock. Naturally I reflashed stock to unroot, but when that didn't fix it I did a full factory reset. The modem was still not working, so I even went all the way and did a NAND erase all + PIT + clear EFS in Odin. The modem still did not work!
3. EFS Professional could not connect to the modem ("send SPC" didn't work at all, rejected by phone error). In QPST I got "disk error while writing to file" error when trying to backup the qcn. In retrospect, these methods probably failed because the modem had been failing to initialize at boot, so these tools weren't able to communicate with the modem. Also note that the Service Mode menu was completely blank, but the USB menu to enable modem diagnostics still worked.
At this point I was close to giving up and about to start finding a mobile phone repair shop who could fix this (I was thinking that I needed a z3x box), until I came across some threads which suggested that they fixed their unknown baseband problem by erasing modemst1 and modemst2. I didn't want to try this because I read somewhere that recent newer Samsung devices stored the IMEI in these partitions, and NOT in the EFS. (don't think this is true anymore...) I had already tried clearing the EFS in #2.
Whilst most of these threads were about non-Samsung devices (they used fastboot erase modemst1, but we don't have fastboot on Samsung), it can still be done in TWRP, here>>>https://www.theandroidsoul.com/fix-e...g-twrp-backup/
After erasing modemst1/modemst2, I rebooted, and WOW my modem now works! SIM is detected!! calls and messages working splendidly☺☺☺
So... now I'm confused why people suggest to backup/restore the modemst1/modemst2 partitions in the first place, when erasing those partitions actually solved my problem - and without loosing my IMEI number/BT or Wi-fi MAC! EFS Professional and QPST tools now work as expected. I now wonder where the IMEI number really is stored?
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Hope this helps someone!
Notes.......
(NAND erase all doesn't actually clear everything. Perhaps there are multiple NAND chips or something)
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