You have control room out jacks and headphones jacks, both controlled by the same panel knob. So if you turn that knob to zero, the phones are still noisy? Seems like what you report. SO are the control room outs also noisy? COnnect each of them to an amp and see. Oh, and are both sides of the phones noisy or just left or right?
WHite noise or hiss is probably a noisy op amp.
If the control room outs are also noisy, then IC48 is probably noisy.
If the CR is clean, and only the phones, then IC1 drives the left phone, and IC2 the right earphone. I mean the sides in your phones, not the two jacks. The jacks are parallel.
The dead channels? Each channel has an Insert jack, which is like a one-jack effects loop. The cutout contacts in them get dirty. Pick a dead channel, put something to its input and connect the mixer to an amp for a listen. Or use phones. Whatever it takes to hear the problem. Now take a plug from something and push it in and out of the insert jack for that channel. Does it wake up that channel, or at least try to? If so, spray some cleaner down the jack and shove a plug in and out to spread it around in there. ANy help?
ANd you can isolate the problem with this jack. Put a signal to the input. Now a cord from the insert to some other jack shopuld allow you to hear that signal if the input circuits work.
The insert is a TRS jack - often called a stereo jack, like for headphones. All the way in is the tip contact, whish is the insert SEND. If you plpug half way in, to the first click, that is the ring contact, which is the RETURN. You should be able to take your signal and plug it into that insert half way and it should come out the mixer. That test at least tells you if trouble is before or after that jack.
But mostly it is the insert jack itself killing the channels.
This is not a problem of this model only or Behringer only. This happens to any mixer with insert jacks, just as the same thing happens to guitar amps with FX loops.