[Asterisk-Users] audio lost on incoming calls

[snacktime]

I've had this nagging problem ever since I started using asterisk and
have yet to find the source of the problem or a solution.

When making calls to DID's that I have from various providers that
terminate into my asterisk box,  The first 1-2 seconds of audio is
lost.  If it was a delay I could understand, but the first seconds of
audio  is just never heard at all.   The only common item I can
identify is that all of these calls go over the pstn at some point. 
When making pure voip calls the audio is just fine.  For example
calling in using a direct sip or iax2 url works fine, as does using my
FWD accounts.

It doesn't seem to matter how good the connection is to my provider. 
It happens equally whether asterisk is running at our data center at
internap or from my dsl at home or at work.  Whether nat is in the
loop somewhere makes no difference either.

Any ideas on what could be causing this, or how to go about debugging it?

Chris