Q: Bad voice call quality

Howdy Adam,

Welcome to Apple Support Communities.

From what I gather, there's a sound quality issue with your iPhone 5s that's occasionally preventing you from hearing the the person that you are calling. The article below provides a lot of great information and troubleshooting tips that should help you resolve this issue. 

If you can't hear a person on a call or voicemail or if the sound quality is poor on iPhone - Apple Support

So long.

No problem.