iPhone 5c volume control is erratic; it turns itself down at irregular times, even during a phone call. I cannot hear it ring OR hear what the caller is saying!! Help!!

I have had an iPhone 5c for a little over a year now. I have had no problems with it at all until about 2 months ago. The ringer turns itself down at irregular intervals and even during some phone calls. I have to keep turning the volume back up just to hear what the caller is saying. This is extremely annoying. I have missed quite a few text messages and phone calls due to this issue.

It is NOT my iPhone case; I have had the case as long as I've had the phone, and this was never a problem until 2 months ago. Also, the phone does the same thing whether it is in a case or naked and out of the case.

I have tried the "old fix" of going into Settings and Music and making sure the Volume was set to OFF; in any case, it is set to MAX.

I'm about ready to switch to a Galaxy or Windows phone; I've had quite enough of missing phone calls and text messages and not even being able to hear what a caller is saying to me during a phone call. What the HECK??

I deeply appreciate any help with this issue.