[Galaxy S 5 Q&A, Help & Troubleshooting] FNetV1:
I am having a weird issue that is baffling me since I got the Samsung Galaxy S5 Triband Sprint phone that I purchased (new old stock) from ebay and activated it under my Sprint plan. The problem is that calls would auto answer when I have some time that I dont use my phone, sometimes when I am using my phone like reading some articles I would notice someone call me and the call would sometimes auto answer and a small rectangular popup window will appear either on the lower right or somewhere upper indicated a call is answer followed by counting seconds. The weird thing about this is that the call appears to be "auto answered" but the calling party only hears ringing until voicemail picks up. Based on my testing, if I call my own phone (using my house phone or another phone) I am able to reproduce the problem just initially, when I saw the call auto answer I would simply hit "End" to end the call, the call will promptly go to my sprint voicemail, but if I call myself again then the phone will not auto answer and will actually ring, and if I pick it up normally then then call will get thru and I can hear myself and everything. From that point over if I call myself right away the phone will ring normally and will not auto answer, I can repeat this like 10 times in a row and the phone will not auto answer like it fixed itself, but if I place my phone on my belt holder and just wait a while, like a few minutes and then I call my phone again then the problem will re-materialize again, on my house phone that I was using to call my phone I can hear it ringing, but the Galaxy S5 is actually not ringing, after waiting like 4 rings (just halftime before going to voicemail) I quickly open my phone, I see no evidence of any call, only after I swipe down to take a look at my status bar do I see a notification that there is an answered call followed by seconds running as if the call is in actual progression, then the call finally goes to voicemail on its own.

By the way the phone is running Lollipop and came with Lollipop already factory installed

The things that I have tried so far:

I went to Settings > Call > Answering/ending calls > and did confirmed that everything, well all 4 boxes are unchecked as it should be since I have never even gone there previously.

>> Call Rejection > Turned it "off" (just in case if that was interfering with anything)

>> Call Accessories > Automatic Answering's checkbox was unchecked and I left it unchecked

>> Automatic answer timer > gives the following three options: 2 seconds, 5 seconds and 10 seconds, I changed it to 10 seconds to attempt to force the glitch to at least wait 10 seconds before auto answering the calls when it feels like doing it, but nope didn't work, phone doesn't vibrate or ring when the issue materializes.

So, settings wise, there is nothing enabled here that is supposed to auto answer any calls.

I did google and someone said that entering into engineering mode and disabling auto answer there works, but the problem is that on my phone I was not able to find the engineering mode and the apps that I installed to open up engineering mode complained that engineering files were not found. The person that was able to resolve his Galaxy S5 auto answer issue was running Kitkat, so maybe Samsung further hid the engineering mode on lollipop is my only guess.

So, I am at a loss here, and I really dont want to install a custom ROM to see if the issue goes away since historically speaking most of the custom roms that I have seen for the other devices I have owned removes the ability to internationally roam and I have a world phone right now and I need to retain the ability to roam internationally when I go traveling so that would mean remaining with Stock.

* I have rooted my phone so any apps that can help with this issue and needs root shouldn't be a problem.