asterisk users - Feb 2005 - More asymmetrical call quality discussion

I've watched the dialogue about how asterisk has to manipulate the 
packets from an IAX2 connection to a SIP client. That said, I'm 
wondering if a previous problem I've been trying to diagnose could be 
related to that process. In short, here's how I describe it:

Outbound:
SIP 7960 > Asterisk > IAX2
Audio is perfect both directions.

Inbound:
IAX2 > Asterisk > SIP 7960 or
SIP > Asterisk > SIP 7960
Caller hears me perfectly.
I hear caller with extreme choppiness. Can make out ~30% of 
conversation.

I've tested this with two VoIP providers, same issue. Tested an IAX2 
DID from a different area code and THE PROBLEM RESOLVES ITSELF. Others 
on this list have reported similar issues which seem to point to the 
CLEC who is bridging the call to IP. (I assume that's how it works? 
Someone please correct me?) Using ulaw all around. ZTDUMMY is loaded 
and modprobe returns nothing.

HYPOTHESIS:
A. Could there simply be a configuration issue with the VoIP provider 
where they are interfacing with the PSTN, and thus those calls are 
requesting some sort of jitter or trunking protocol that I can't 
handle? This seems to be less likely, though, because I can reproduce 
the problem by registering the 7960 directly to the provider, bypassing 
all my hardware; even my topmost switch in my network.

OR

B. Does anyone think it is in fact a problem with how the CLEC is 
giving the call to the VoIP provider? Apparently all DIDs in this area 
are serviced by the same CLEC. Does anyone have any theory on what 
could cause this?

Thanks,
/rg