No sound when I get called



I bought a new phone like a week ago, the Lenovo (Vibe) C2 K10a40 running Android 6.0, and the phone works perfectly fine, except for one thing. As you read in the title when someone calls me (yeah, people still use that function of the phone) there's no sound, even though the volume is set to max. It's only vibrating. Also, after I reboot the phone the problem seems to be fixed, but after an hour or two it comes back again. Another weird thing is that if I change the ringtone and then reboot it goes back to the default one. I tried factory resetting but obviously that didn't work. Any ideas on how to fix it or should I just return it and get another one?