Why does my iPhone keep dropping calls without actually hanging up on caller?

So I’ve had my iPhone 6s for several months now and the system has seemed to be malfunctioned from the beginning. I don’t think I purchased a refurnished phone either. Anyway I’ll be talking on the phone, typically another iPhone user, and I’ll be mid-sentence and all of the sudden the person I am speaking to can’t hear me anymore. The call is still perfectly connected and I can hear every word they are saying, so I just have to hang up the call and call them back. One time when this happened it actually just dropped the call and a message appeared on my phone screen saying ‘system failure’ that I had never seen before. My dad spoke to someone already who said it could be my Bluetooth connection because I always had it on, connected to my fitbit and use it often to connect wirelessly to speakers to listen to music. I no longer leave my Bluetooth function on, which seemed to help at first until it inevitably started doing the same thing. I’ve updated my phone, I’ve also disconnected my old iPhone 5 that was somehow linked to it and my iPad, thinking that was causing the problem. Do you think this can be fixed? Could it be the speaker on my device?