Bad voice quality on phone calls, good with WhatsApp
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!