Dumb question.. no sound through headphones/jack?

As the title states, I've got what's likely a dumb question. Plugging in my favorite set of headphones, (which work with every other portable device I've tried them on), doesn't work.. the sound continues to play through the phone speaker instead. Is there some silly setting I need to fiddle with to enable the headphone jack or something? As always, thanks in advance!

 

That was the first place I checked.. but I see nothing having to do with the headphone jack/volume for such/external etc. I've run into a setting somewhere that enabled external sound when docked or whatever, and tried it, to no avail. (obviously docking stations don't utilize the 3.5mm jack, but figured it was worth a shot) FWIW, my phones from Verizon.

 

Now I have, lol.. works fine with the included earbuds. WTF though, like I said, my Denon set work fine with every other 3.5mm jack device I've ever used them through, mobile or not. (even my old junk flip-phone) Is there something special I need to look for? I hate earbuds/prefer around the ear type & want really good sound quality.