Originally Posted by waranty92
I'm not getting answer of main question, can I flash custom rom or root Samsung SM-G900A and use it with any sim card even if it is blacklisted by AT&T
If your phone is locked you cannot use any other SIM card but the one that is for it! IE an ATT locked phone will only work CORRECTLY with an ATT SIM card or straight talk SIM card for ATT.
In order to FLASH a NEW ROM (Operating system) You have to Flash a new Bootloader IE ROOT your Phone. This will enable many things and give you more power IE delete bloatware, flash a new OS, etc. The Bootloader on the S5 is encrypted but many have found ways to replace it on the S5. If you find a good ROOT for the S5(ATT) use it at your own risk!
It is doable but may brick your phone if you do it incorrectly!
You can buy an unlock code from many places. Search for 'Unlock Galaxy S5'