How can I improve my freepbx and

I configured my freepbx sip trunk as it says at:

I’m also behind a proxy: EdgeMarc 4500T4 that should not be a problem,
because I was using another windows PBX without problems (same config as

The hardware is dual Opteron 2GHz with 2Gb ram and SATA drives. (top shows:
very low ram % used, very low cpu % use…)

All works fine and audio is perfect, but when I dial out using trunk I hear cut and noisy voices. It sounds like very
low quality audio.
My internet conection is simetric 10Mb/s, so I don’t think its a problem.

Any idea, tip or sugestion about how to improve the audio performance?