asterisk users - Aug 2004 - dropouts

hi all

I keep getting 'bzzt'-sounding dropouts, but I've only heard them
when
from behind NAT. I don't think it's related to bandwidth, as I'm on
a
2400/640 ADSL line. I'm currently using ALAW. Any ideas how to debug 
this? SIP DEBUG doesn't show me anything when dropouts happen, although 
I'd doubted it'd do so... How can I debug the RTP part of SIP?

roy