When making or receiving a call I can hear the person but they cannot hear me?

Try calling through Whatsapp, Facebook Messenger, Skype or any other Internet Calling, this will check if the problem is with Fizz phone calling system or your mobile.
As these apps uses internet to call, they will not use Fizz... you can try even without the simcard inserted using some WiFi Internet.

Where are you residing, maybe a weak signal?

Very weird, does it work when you put it on speaker? Or with hands-free?

Move your sim to another phone and give it a try.My guess is that there is some dirt accumulated in Iphone microphone.

Ok I tried with with earbuds with integrated mic and it works perfectly and it also works well when I am on speaker phone. So still not sure how to fix this but at least I know it is not because of FIZZ network but probably because of my crappy iphone 6.
Thank you for your help guys!