I realize that this is an old thread, but here is an idea for you. As far as I know, pretty much any 2.5mm headset should work. I don't use it for my phone personally, but I recently purchased a Plantronics Gamecon X20
headset for my Xbox 360 after my old one broke. Since it has a plain 2.5mm jack on the end, I thought I would plug it into my phone to see how well it worked. The answer, pretty well apparently. My wife said that it was really clear and, likewise, she sounded really clear on my end. It's going to be a little larger than the ones that have the mic hanging down inline with the wire, but on the other hand, since it's a boom mic the sound quality should be superior with less noise. I haven't tried it for listening to music so I can't vouch for the quality there, but since you are wanting a monaural headset I rather doubt that is relevant anyways. The headset was only $20 at GameStop and they had a used one for $10.
Originally Posted by two-zero
I had a similar problem (not the same). After using a 2.5 to 3.5 mm adapter my phone speakers and the mic didn't work anymore. I was about to send the device in, when I read about a solution (I think it was in XDA-Dev somewhere).
You'll have to gently move the headphone jack forward and back, twisting it a litte or even better use some compressed air blowing into the jack. It works best, if you to take out the battery and blow from the back.
My speakers work again, but the same thing happens now and then. It seems there is some switch inside the headphone/mic jack, which doesn't go back. That only happens when I use the adapter.
In your case, I would say that sounds like a bad adapter. I routinely use one in mine and don't have that problem. It's one of those nasty stick adapters though, so you have to be careful with them. I've seen some cable type adapters for not too much so you might look into one of those instead.