[Android Q&A, Help & Troubleshooting] asg001:
Hi all!

I bought an S6 Edge second hand and after flashing a stock ROM, I had the "emergency only" calls issue, I tried everything I could find in the forums to no avail and I ended up taking it to a repair service. I did not think they could fix it but they did and I would like to know what they did to it in order to prevent it from happening again and to know if I can leave it as default (not custom OS) so that it can upgrade.

State of phone when bought:

So, the phone was working well with all carriers (from Colombia). The only thing I was able to notice after I bought it was that it was still on Marshmallow (6.0.1) with carrier branded stuff. It was in perfect shape otherwise.

So I downloaded a stock ROM and installed it using Odin. After this the phone would not let me make calls as it was not registering on the network. It did not even let me make emergency calls despite getting full bars of signal and being able to recognise the list of carriers when attempting to manually connect to them.

Important: The ROM downloaded was for the right version of the phone which was the G925i and the IMEI, serial number and baseband were not altered from the original. Also, I checked the phone to see if it was reported as lost/stolen and it was not (of this I'm sure because in Colombia you can consult a centralised database on any carrier's webpage). Also.. Here in Colombia it is illegal for carriers to lock phones.

Steps I took to fix it:

Downloaded ROMs for all carriers, branded and unbranded, I even installed one from Trinidad and Tobago and a LATAM one.

I performed all the tricks to disable and enable flight mode, location services, try con connect manually to the networks, changing to GSM only, GMS/CDMA, international roaming, etc.

I also extracted modem.bin from an earlier version and flashed the modem.bin with Odin (6.0.1 moden on 7.0, 5.1.1 moden on 6.0.1 and 5.1.1 moden on 7.0). All modems flashed correctly and the issue remained exactly the same.

So I finally gave up.. I installed the latest stock ROM which was an unbranded Nougat (7.0) version and took it to a repair service and they fixed it in a couple hours.

What I noticed after the phone was "repaired":

- The phone was rooted, SuperSu was installed, no other root app was installed.

- On SuperSu, there are entries for ADB shell (they all say Superuser command contents have not been logged for this entry) in a two minute timespan.

- The android version was downgraded to 6.0.1, I was told not to try to update it.

- Developer options was enabled with USB debugging turned on.

- The boot option (the one where you can wipe data and cache partition) was left as original (not the TWRP one).

- The ROM used to downgrade was the same I had before but only downgraded the version.

I suspect this same procedure was performed before on the phone but it had the original ROM carrier branded so it seems as if they installed a custom ROM, rooted it, connected with pc and performed some command line stuff to fix it, then cleaned the bootloader to a stock one.

The questions:

Any ideas what did the repairman do to fix the phone? Also, would it be safe to make a full backup with TWRP and flash again a stock ROM upgraded to 7.0 and then flash the backed up modem to get it working? Or would that possibly screw up the repair?

Thank you all in advance for any help. I'm sure there are lots of people out there experiencing the same "emergency only" issue I had but there does not seem to be a fix around, but clearly there is one! So I hope this sheds some light into what could possibly happen.