Audio Systems: Ringtone quality, frequency response curves, frequency response curve

Question
I just got frequency response of my mobile phone speaker. I want improve mp3 ringtone sound created with Audacity, to make it sound better. Here is frequency response curve:
http://img94.imageshack.us/img94/5353/frequencyr.jpg

I just not fully understand the diagram: why Magnitude dB (on Y-axis) have positive values?  Does this mean signal amplification? I just usually saw frequency response curves with Gain/Loss (dB) from 0 to -40 specified in Y-axis.
What means "Magnitude dB re 20.00mPa/V"?

What is low frequency which can be safely cut off in Audacity High-Pass Filter for better music sound on my mobile phone?

Best Regards

Answer
The graph is just telling you the levels of every frequency in the audible range. I can't tell you what to boost or cut as I cannot hear what you are hearing. It's just going to take experimentation. I do see a pronounced boost from 5k-20k though. These being all high frequencies. Your phone will not be able to carry low frequencies so you probably aren't them anyway. Just toy with it and see what you get.