My Samsung S7, with Android 7.0 (modified by/for T-mobile) stops ringing.

[Android Central Question]

When a call comes in, I get about 2 seconds of the ringtone, and it just stops ringing. This started happening a few days ago. I switched from the Zedge ringtones I was using, to the standard/default one and nothing changed.
I performed the cache wipe, just in case. No difference.
This morning my alarm did the same thing for the first time, and it keeps on doing it regardless of the time I choose for it. Interestingly, for a few days about 2 weeks ago, the same problem used to occur when playing back video attachments, but that fixed itself somehow.

 

[B. Diddy]

Welcome to Android Central! Does this also happen in Safe Mode? https://support.t-mobile.com/docs/DOC-28008

 

[JnArango]

Safe mode made no difference either.

 

[B. Diddy]

You could try reinstalling the firmware using Smart Switch: http://www.sammobile.com/forum/showthread.php?t=29914. Make sure you backup or sync your data beforehand.

 

[JnArango]

That is a bit drastic, but I may end up having to do it.
I just stumbled onto something interesting: If I am using headphones, the ringtone works flawlessly. It gets more interesting: If I wait until the ringtone dies out on the speaker, then connect the headphones and it can be heard on both speaker and headphones, I can disconnect the headphones and the ringtone continues on the speaker!

 

[B. Diddy]

That is odd. The fact that it persists in Safe Mode suggests it's either a firmware or hardware issue, which is why I suggested reinstalling the firmware as your next step. If it were due to some interfering 3rd party app, it should've gone away in Safe Mode, and one would hope that a preinstalled app wouldn't be so messed up that it would cause this problem.

A hardware issue with the headphone jack also seems unlikely, since it doesn't happen right away -- the speaker seems to work initially.

 

[JnArango]

I am sure it is not hardware. I can have a long call using the phone as a speaker phone with no issues whatsoever. I can shake the phone, plug and unplug the headset or connect/disconnect Bluetooth, and the audio is flawless. Likewise, I can play music back without any issues.
I changed the settings so the phone vibrates while it rings, and disabled the option to mute the ringing with a gesture. The vibration continues even when the ringtone stops.
I had not used T-Mobile's Visual Voicemail for a while simply because I had no recent messages. I used it today, and voila! The audio when listening to a message stops a couple of seconds in, just as it used to do a few weeks ago with attachments. Now that I know the trick, I can plug and then unplug the headset to get audio back on the speaker.
So: Not hardware, not limited to ringtone (alarm, notifications and playback through certain apps are affected too), and it persists in safe mode. I disabled Visual Voice Mail, just in case. No improvement.
It has to be audio drivers.