Hello xlannex,
Thank you for using Apple Support Communities!
I understand from your post that when joining a call or answering a call you hear distorted sound and you cannot be heard. Also, when you hang up and try to call back the call fails. This article has steps that address distorted sound issues which you may find helpful:
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support
This one has steps which address what to do if you cannot make or receive calls:
If you can't make or receive calls on your iPhone - Apple Support
Best Regards.