Might Just Need A Needle
I know this might sound like a stupid/common sense comment, but after failing many different attempts at this, I had an "AH-HAH" moment. This was when i heard a mild distortion, when i pressed my ear firmly against the phone.
So, I took a needle, and started running it across the ear/call speaker. I immediately started noticing some crap getting scraped off (phone is 1.5yo, after all). This, in spite of it looking 100% clean. I did this for a bit, using a dust buster after every few passes. That way, I didn't press anything farther in & suctioned out, what I couldn't access. Do this until nothing seems to come off anymore.
You really don't need to worry about damaging the speaker, as you're only cleaning the protective plastic shield/guard. The holes are extremely small in this plastic cover. Way smaller than the width of a needle. You may want to go after the individual holes, if you don't get enough increased volume.
---------- Post added at 01:57 AM ---------- Previous post was at 01:45 AM ----------
Also, in addition to the needle method above, if the benefits aren't significant enough, can bring in the artillery:
Canned Air (with the pressure multiplying straw)
Powerful Carpet Vacuum Hose- And if needed, narrow the suction beam, by placing something over the hose opening. Be it your fingers, or even an old piece of plastic with a hole cut into it.
Remember to use the needle method first, or during above methods. Since, if the crud isn't loosened first, it's not going to be moving anywhere.