Have the same problem. Callers can't hear me, I can't hear them. Doesn't work with phone speaker and mic, doesn't work with Bluetooth, doesn't work with headphones. I can hear music via Bluetooth, I can hear sounds on my phone, I can hear music through speaker. Can also hear and record voicemails. Only thing that doesn't work is phone calls. Have no problems with data, texting, anything.
Went to Apple and they said the microphone is out. Had no comment why I can make phone calls like normal on Facebook Messenger and all voicemail functions work. No explanation why microphone in the phone keeps me from making calls while on Bluetooth.
Went to AT&T and they said it's a phone problem. Sim card makes calls in other phones.