Exclamation [Q] No sound on calls, won't hang up, only SOMETIMES

I've searched and can't find anything similar with this phone.
Problem started randomly and occurs randomly. Sometimes, when I make or receive a call, I will get no sound whatsoever. Not through the earpiece, speakerphone won't do it, never have connected a bluetooth etc. Also, when it happens, I can't get the phone to "hang up" either. It's like the phone call is stuck. It happened when the phone was on Gingerbread, but I upgraded to 4.0.4 and it still does the same thing. No custom roms or anything, but I wouldn't be opposed to it as a fix. Is it a hardware thing? Help!