iPhone 5S

I've had my iPhone 5S for going on 6 months now. Prior to the 5S I had the 4S. I'd purchased a bluetooth headset that worked fantastic with the 4S. After getting the 5S and pairing it (successfully) callers indicated that the sound quality was terrible. So I bought another headset to use with the 5S. Paired that one successfully. I still had people complaining about the sound quality so I took the phone to the Apple Store and they gave me a new phone.

I've now gone through multiple phones and multiple headsets and I can't figure out why the audio is so terrible for callers. It's without fail, everyone complains if I use it. No matter the cell phone provider they're using, where I am calling from, etc.

Any recommendations on either how to fix this or for a good headset that won't cost hundreds of dollars? The ones I've been using have been in the $60-70 range.

Thank you!!