I recently experienced this issue as well. It started out that I couldn't hear my callers, and my callers couldn't hear me and then I lost all functionality to make/accept calls and my callers were expressing that my phone was going straight to voicemail.
I followed the steps in this article: https://support.apple.com/en-us/HT204027
I turned airplane mode on/off and that solved the issue of not being able to make/accept calls, but I was still unable to hear my callers and vice versa. I tried most of the steps outlined in this article, but the step that worked for me was resetting my network settings.
I hope this helps!