Problem with iPhone 6 when calling

Hello,

When someone calls me or when I call someone through my iPhone 6 I can't hear any voice and when I press the volume up button, it said headphones instead of voice even though I did not plug any earpod. But when I plugged in my earpod or used the speaker I can hear the voice clearly. I did go to an iPhone store and they did fix the problem but then it happened again