Caller on the other side of the phone cannot hear me, does anyone know how this could be resolved?

I have an iPhone 6 and its always had its iOS kept up to date.

When I am on the phone the person on the other end of the phone cannot hear me but I can hear them perfectly fine.

I have taken my phone to apple and they have run every test they can think off and they can't find anything wrong with my phone, they checked all the microphones and they are all working fine. The phone was set back to factory settings and and the issue was still apparent (which suggests it isn't a software issue) . Their is no fault with the sim and this issue only happens during a phone call and not on facetime or whatsapp call or facetime audio. If I cannot find a way to fix this I will have to get a new phone but I would rather just try to fix it.

I would like to hear if this has happened to anyone else and if you know how to fix it.