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?