Bad voice quality on phone calls, good with WhatsApp I have two Samsung Galaxy S5 minis, one of which has the following problem:

The voice quality is really bad on phone calls to other people (muffled sound, chopped, extra noise/interference, echo). 

I suspect it has nothing to do with the network strength, as the voice quality of the other device is very good in the same spots.

To rule out hardware issues, I called other people via WhatsApp. Everything is just fine there. Also voice notes on WhatsApp do not show any problems. So I would suspect that the microphone is ok.

I must admit that I did not try to reset the phone yet (factory reset), as I would not like to spend the time on reconfiguring everything to my needs - just to find out the problem persist.

So is there anything I can do before trying a factory reset or even send the phone in for inspection? Is there any setting that I can change? I removed some of the preinstalled apps, could this be an issue? Or is there another caller/phone app that I can try?
I tried a factory reset to resolve the problem, but it did not help.
Also I send in the phone to the sellers technical service. They wanted more money for repairing the phone than the phone is worth.
However, there were suggestions on the internet stating that sending the device to Samsung could help. Even if they would not repair the device, it could help resolve the problem. However, it turned out that my phone had still warranty, so they repaired it for no cost! Now everything is working fine!