Samsung Galaxy S5 — I can't make a phone call or receive a call without a headset

I can’t receive a phone call or make call with my Galaxy S5 unless I use a headset.
If I don’t have headset connected to the phone, when I try to receieve the call or make a phone call, the screen freezes until the call fails (the samething happens for even accessing voice message).
I called my service provider and the rep said it’s the device issue and has nothing to do with the signal since the phone has no problem getting the signal with the headset on.
I tried to find settings that might affected this and searched for similar problems but I have yet to find what's triggered this.
Disabling bluetooth (I don't even have it on, I tried enable it, then disabled it), deleting cache partition and even factory reset didn’t help it.
There are times I really need to make a call without headset on, I would like to know if there's any solution to this.