microphone and speaker don't work ONLY when calling

I am in possession of iphone 6 and when I do normal calls I cannot neither hear, nor the other side can hear me but here is a currious thing, if I use facetime, whatsapp, telegram or any other app the mic and the speaker are working perfectly. It is not a matter of provider as I already tried different SIM cards and the result is the same. Also, naturally, I tried to restart and it did not fixed the problem.